Simon Calder: Why squander slots on the shortest hops?

The man who pays his way

Open skies: a new view from Terminal 4 (Douglas Bolton)

As I prepared to fly from Heathrow to Paris CDG, I pondered why anyone would fly from Heathrow to Paris CDG. Twenty years ago, they were at either end of the world's busiest international air route. British Airways even launched its new Boeing 777 on the link between the English and French capitals.

Then Eurostar got into its stride, and the default means of travel between Paris and London involved burrowing under, rather than flying over, the Channel. While neither St Pancras in London nor Gare du Nord in Paris is quite as central as the train operator would like to claim, they are both much handier for the average tourist or business traveller than Heathrow or Charles de Gaulle airport. And even with the half-hour check-in Eurostar demands, the journey time by train is under three hours. Air France insists on a 40-minute check-in followed by an 80-minute journey, making two hours airport-to-airport before you add the journey to Heathrow and from Charles de Gaulle.

So, why does the French airline, and British Airways, expend valuable slots on this link? Each shuttle eight times a day, both ways, between LHR and CDG. The reason: connections.

BA wants to feed its long-haul services from Heathrow. Paris, being the biggest city in the western part of Continental Europe, is a prime candidate. So from early morning to last thing at night, BA has Airbuses airborne between the two airports (and even adds three more to and from Orly in southern Paris, which taps into a different catchment area in the French capital).

Air France is even more dependent on transfer traffic than is BA. With London being the biggest aviation market in the world, the easiest way to fill long-haul seats from Paris CDG is to fly in passengers from Heathrow. People are prepared to trade time and convenience to save a couple of hundred pounds. I am one such traveller, and that was why I was flying from Heathrow to Paris.

Terminal velocity

It didn't begin well. Air France has the misfortune to fly from Heathrow Terminal 4, the least-loved and worst-connected facility at Britain's biggest airport. It was always an unwanted child – a sub-optimal stop-gap while permission was sought to build Terminal 5. While "T5" sits neatly between Heathrow's two runways, T4 is a forlorn outpost shoe-horned between the airport's southern runway and the A30 trunk road.

The good news: Terminal 4 is served by direct trains from London's Paddington station, taking half-an-hour for a fare of £9.90. The bad news: there are only three a day: at 4.42am, 5.13am and 11.07pm. Should none of the trio suit your travel plans, you are invited to pay £21 and change trains en route, or risk the cheaper Piccadilly Line from central London. But the dark blue Tube line unravels at its western end, with frayed ends serving several destinations. In my experience Terminal 4 is served by even fewer Piccadilly Line trains than Planet Tharg.

If and when you reach the terminal, you will probably find the Paris flight at Gate 15. A long trudge – but with a miraculous addendum.

Right next to the gate, a set of stairs leads up to View Heathrow – an observation tower, furnished with binoculars. From this free facility, you watch the hypnotic cycle of landings and take-offs. To the east, London's spiky skyline punctuates the horizon. From the moment I saw the panorama, T4 leapt from worst to first.

On board the plane, its absurd location became clear as we waited to cross the busiest runway for landings in the world. Yet more aesthetic treats awaited in the sky. As the patchwork of English meadows softened by summer retreated, we flew over Seven Sisters (the dramatic Sussex cliffs, not the scruffy north London suburb). On the French side of the Channel, the rural geometry changed to a more angular pattern, disrupted by the odd swish of high-speed railway. On the approach to Paris, we found ourselves racing a high-speed train south.

It was such a compelling journey that I almost failed to notice that Air France's catering seems intended to insult the British and French equally: the former with an undrinkable approximation to tea, the latter with flabby, stale croissants.

Coming back, the catering was equally contemptible and the journey even more protracted. On a reasonably direct track, the 200-mile Paris-Heathrow flight should take 40 minutes. But by the time we had flown circuits over the Home Counties and queued to cross a hyperactive runway, it took a full hour longer.

While arguments about airport capacity rage, it might seem barmy to have 1,000 flights a month hopping between Heathrow and Paris CDG. Yet they are not going to stop any time soon; the route still makes the list of top 10 destinations from Heathrow by passenger numbers. Regulators regulate, but the market decides.

In Lust, in Bedfont

Faced with an eight-hour wait for the next direct train, I sought an escape route from Terminal 4. Aim south-east across the A30 and through a thicket, and you find yourself in bucolic surroundings striding beside the Duke of Northumberland's River. Ducks carve aquatic tracks, while ponies whinny in a field beyond.

Waterways lead to Feltham railway station, but along the way you encounter Bedfont Village – rich in replenishment options for passengers who declined Air France's inflight catering. I was particularly taken by In Lust: not a retailer specialising in what used to be known quaintly as marital aids, but an "Urban Indian" restaurant. Its speciality, the Hyderabad biryani, costs under £8, including a proper cup of tea.