A sign on the waterfront at Weedon was unequivocal. 'This is a working boatyard', it cautioned the row of modern houses across the canal. 'If you do not like the view, the boats or the noises we make in the course of our business, please do not come and live opposite then complain'.

Britain's canals were not built for recreation. Two centuries ago they were its 'motorways', the lifelines of a growing economic infrastructure as speculators raced to cash in on the gains of the Industrial Revolution. But road and rail have long since taken over, and the canals' chief function nowadays is that of relaxation. This requires boats, however, and boats have to be built, and the conflict between work and leisure lives on in Weedon, Northamptonshire.

The Grand Junction canal was begun in 1793, when a third of the country's network was being dug. It ran from Brentford on the Thames to Braunston in Northamptonshire, joining other waterways to link London with all the Midlands cities. In 1929 the various canal companies merged to create the Grand Union Canal, a single waterway between London and Birmingham.

Canals predated steam power, so early boats were towed by horses. Next to the canals, more than 1,500 miles of towpaths were built, which survive as havens for wildlife and attractive paths for walkers.

To mark the bicentenary of the Grand Junction, British Waterways opened the Grand Union Canal Walk last July, following the towpath from London's Little Venice to Birmingham's Gas Street Basin. It is, in a sense, Britain's first purpose-built long-distance trail. As Bruce Harding, British Waterways' recreation manager, points out: 'Towpaths are virtually the only rural footpaths in Britain that were actually designed to be walked on.' They are also reliably flat, apart from short climbs at locks.

It would take a serious walker about a week to cover the full 145 miles, through the greenery of Milton Keynes, the historic towns of Warwick and Leamington Spa, the commuter country around Watford and Hemel Hempstead . . . with a great city at either end. Most of the path is not officially a public right of way, but do not be put off: British Waterways is keen to point out that it encourages walkers. It even issues a 'passport' which you can get stamped at 10 points along the route to prove you have done it.

With two days at my disposal I decided to concentrate on a rural section in Northamptonshire. I started at Braunston, once England's largest inland port and still a regional canal centre, with a British Waterways office and exhibition room. From the twin iron bridges at the junction with the Oxford Canal, I headed east past a series of locks. In August the canal was alive with holiday traffic, gaily painted narrowboats with roses and castles insignia, their roofs adorned with flowers. I watched a rare working boat filled with sacks of coal negotiate the bottom lock, followed by a narrowboat named Narroway, with biblical references painted on its side.

The canal passed under several bridges before entering the mile-long Braunston Tunnel. While it went under, I went over. Only the occasional ventilation shaft, protruding awkwardly into a field of hay, hinted at the presence of boats 100ft below. I could have walked this patch of countryside quite unaware of the canal.

I crossed a road - one of only three that interrupt the walk between London and Birmingham - and rejoined the towpath where it was being repaired by a party of conservation volunteers. Boats were moored all along the banks as I approached Norton Junction, where an arm of the canal leads off to Leicester. I watched one boatman polishing his brasses, another busily twisting rope, and a woman painting her mop handle while a dog dozed at her feet.

Between Long Buckby and Weedon the canal runs parallel to the M1, and at one point there was only a hedge between me and the speeding juggernauts; I could not fail to notice the noise, but the drivers raced by, ignorant of the canal and the boats. Across the water, InterCity trains sped in and out of tunnels, separated from the towpath by a canal and a field of cattle. Eighteenth-century canal, 19th- century railway, 20th-century motorway - 300 years of transport history in a few hundred yards.

The motorway receded and the towpath became quiet once more, the haunt of fishermen and cyclists and of a solitary painter. Wild flowers were in abundance and dragonflies buzzed around the hedgerows. In my shorts I was an easy victim for nettles, thorns and insects. Boats cruised by at around 4mph. I overtook them at the frequent locks, and left them behind me. After Weedon I came to Nether Heyford, a distance of 11 miles in just over four hours.

I spent the night at Swingbridge House, a former stabling inn where a boatman could exchange his tired tow-horse for a fresher one. A 'pair' of old working boats (one motorised, the other a 'butty', an old Brummie word for 'mate') sat outside in the water; from my room I looked over the boats and the canal to see a heron swooping on its catch.

On the second morning I walked another 10 miles to Stoke Bruerne. The highlight of this section is the Blisworth Tunnel (Braunston and Blisworth are the only two tunnels along the route, built because the contours made locks impractical and the hills were too wide to be skirted). The tunnel at Blisworth is 3,056 yards long and took a decade to build, using more than 5,000 million bricks.

It was the final link in the London to Birmingham chain: until it was built, goods were horse-drawn along a railway between the two sides. Even after the tunnel opened in 1805 there was the problem of how to get the boats through, there being no towpath. Children led the horses over the top, along the minor road which I followed, while the boat was propelled by a professional 'legger', who lay on a wooden plank kicking furiously against the tunnel sides. The process took about three hours, and most of the leggers died young with rheumatism.

The tunnel ends at Stoke Bruerne, where there is a Canal Museum, based on the collection of a former lock-keeper and housed in a converted cornmill. Stoke Bruerne is likely to be packed with schoolchildren in term-time and with tourists in summer, but for the tired walker there is the compensation of a pub and several restaurants and tea rooms. It is also a base for pleasure cruises into the tunnel or through the set of seven locks.

Throughout the summer, Bruce Harding gives a series of guided walks along different sections of the canal, and I timed my arrival in Stoke Bruerne to coincide with one of these. He is something of an expert, having twice walked the length of the canal, cycled it, and covered it by boat and helicopter. It was he who first planned, then developed, the idea of the London to Birmingham walk.

For two hours he talked enthusiastically about his beloved canal, guiding us through its history and ecology, the lives of the boat people, the workings of lock mechanisms, bombarding us with unmanageable statistics while doing his best to make them mean something. 'Every time a boat goes through a lock,' he said, 'it uses 60,000 gallons of water. That's a bath a day every day for four years.'

Ten thousand boats a year through Stoke Bruerne (in its heyday there were three times as many), each going through seven locks - the figures were mind-boggling. All I know is that during our short walk I saw more water disappear than I have used in my lifetime. Does this mean that boats are not as environmentally friendly as I used to think?


Getting there: The Grand Union Canal is served by several railway stations, from Uxbridge to Leamington Spa. Access to the Northamptonshire section is easiest by car - Braunston is on the A45 near Daventry and Stoke Bruerne is on the A508 south of Northampton. The only easy way of travelling between Braunston and Stoke Bruerne is by taxi.

Accommodation: A guide to accommodation is available from British Waterways (pounds 1.50). Tony Kelly stayed at Swingbridge House, Nether Heyford (0327 341383, B&B pounds 55 double).

Books: The Grand Union Canal Walk by Anthony Burton and Neil Curtis (Aurum, pounds 9.99) is essential (with OS maps).

Braunston Boat Show: This weekend (Sat-Mon) the annual show takes place at Braunston Marina, featuring a rally of historic narrowboats. Entrance: adults pounds 2, children/concessions pounds 1. The show is in conjunction with the Braunston Music Festival, highlighted by a Handel concert with real water and fireworks.

Further information: Canal Museum, Stoke Bruerne (0604 862229), open 10am-6pm daily in summer, 10am-4pm Tue-Sun in winter. Adults pounds 2.50, children pounds 1.50, family ticket pounds 6.

Guided Walks start on 1 June and continue until 24 August. Details from: Bruce Harding, British Waterways, The Stop House, Braunston, Northamptonshire NN11 7JQ (0788 890666).

(Photograph omitted)