Osterley Park café, Jersey Road, Isleworth, Middlesex

All too often, the food on family days out is no picnic. Can the café at Osterley Park match its gorgeous grounds?

As an Acton resident with four young children, I'm quite an expert on west London attractions. Scarcely a weekend passes without a trip to the Kew Bridge Steam Museum or Gunnersbury Park. When you're woken at 6am and your eldest doesn't go to bed till 8pm, the days can be long and an activity of some kind is essential.

The food in these "family friendly" attractions is usually filthy – crisps and Kia-Ora for the children, anaemic white-bread sandwiches for mum and dad. However, I've recently discovered an exception to this rule: the café at Osterley Park. In my experience, National Trust restaurants are a cut above average, but this one is particularly good. The basics are fresh and tasty, thanks to the fruit and vegetables supplied by the local tenant farmer, but the really impressive thing is the wealth of home-made produce on sale. The kids' lunchboxes, for instance, always contain a freshly made shortbread biscuit.

Osterley Park is in Hounslow, just off the M4, six miles from Heathrow. In addition to the 16th-century house, there are some well-kept gardens, 142 acres of parkland and a large lake fed by a river. Apart from the jumbo jets passing overheard, it's an idyllic spot and one of west London's most popular destinations. It averages between 300,000 and 400,000 visitors a year.

The restaurant and tearoom is in a Tudor stable block 100 yards from the main house. The serving area is small and there are often queues at the weekend, but it has seating for 80 people, with a courtyard capable of absorbing any overspill. One of my favourite things about the restaurant is a large shelf towards the back containing second-hand books for sale. If a title catches your eye, you simply put whatever you think it's worth in a box.

The menu varies daily, with a firm emphasis on hearty, inexpensive fare. The National Trust has more than 300 properties, making it one of the biggest restaurant chains in the country, and it has strict policies on what can be served. All food has to be British (and that doesn't include chicken tikka masala) and the produce has to be fresh, local and seasonal, as far as possible. That's easily accomplished at Osterley, thanks to the 215 acres of farmland that forms part of the estate. The Sutton family, who've been farming this land for several generations, supply both the restaurant and farm shop.

I'm a regular visitor to Osterley and can recommend the cheese scones and freshly prepared sandwiches. Today, the specials include a home-made mushroom and tarragon tart and a home-made watercress and Wensleydale cheese pie, both of which go down a treat with my wife and me. The home-made treacle tart is good, too. The only disappointment is the Scotch egg, which I order in a fit of greed. The layer of sausage meat is thin and tasteless and the breadcrumbs a bit sawdusty.

The children love their lunchboxes. Apart from the shortbread biscuit, they also contain raisins, crisps and a fresh cheese or ham sandwich. In addition, the children are entitled to a glass of "sugar-free" squash. At £2.95 a box, that's a bargain.

This is fairly unambitious food without airs or graces. "What people come to our restaurants for is a tasty refuelling stop, not a five-star meal," says Ivo Dawnay, the National Trust's communications director.

The Osterley Park café serves 45,000 covers a year, so it can't do anything too fancy. But within these parameters, it does a good job and exemplifies the virtues of the Trust. There's care and dedication and a commitment to maintaining standards, all wrapped up in a friendly, amateurish package. It's quirky and idiosyncratic – as though the manager has decided to include something on the menu just because it's a personal favourite – and all the better for that. The word "nice" isn't one I reach for very often, but it's hard to think of a more appropriate word to describe this place.

As we leave, my six-year-old daughter says, "I hope we're not going to come here every weekend."

"Why not? Don't you like it?"

"I don't like the fact that you always steal my biscuit."

I promise to order my own next time.

15/20

Scores: 1-9 stay home and cook, 10-11 needs help, 12 ok, 13 pleasant enough, 14 good, 15 very good, 16 capable of greatness, 17 special, can't wait to go back, 18 highly honourable, 19 unique and memorable, 20 as good as it gets

Osterley Park café, Jersey Road, Isleworth, Middlesex, tel: 01494 755 566. Call for opening hours. £2.95 per child's lunchbox; £11.10 per adult lunchbox

Second helpings: Day out destinations

Riverford

Wash Barn, near Totnes, Devon, tel: 01803 762 074

A million miles from sackcloth-and-ashes style, this elegant communal-dining shed – on an organic farm – offers fresh food and fresher vegetables. Leave time for a farm tour

Tree House (The Alnwick Garden)

Alnwick, Northumberland, tel: 01665 511 852

The fantastic setting – it really is built around a huge tree – makes this an outstanding destination for a family day out

The Restaurant at St Paul's Cathedral

London EC4, tel: 020 7248 2469

In the impressive crypt of St Paul's, an airy new British restaurant with an unexpected get-away-from-it-all feel; the food doesn't try too hard, but that's all part of the winning package

Reviews extracted from 'Harden's London and UK Restaurant Guides 2010'