Gosh, what a lot of people worried about my safety/legality/insurance!
To set your minds at rest (a) there is no question of my doing it if the insurance won't cover it; (b) the space in question is (without measuring) at least 800m x 400m, possibly larger, and the largest app/dep obstacle would be trees; (c) as it would be done with the landowner's permission and the school is not in a built-up area (not even nearly) there should be no problems with rule 5. I've certainly landed in smaller spaces and on much less well-tended grass. And I wouldn't do it in the rugby season - the posts are the highest in the country apart from Twickenham, apparently, but they are taken down from January to September.
Anyway, thanks to those who answered the question - reassuring. I'll let you know if I actually do it.
Tim
(PS it's been interesting to be on the receiving end of the worried PPrune instant reaction 'oh, you can't do that, it's illegal/dangerous...' when that wasn't the point of the question!)
(PPS Monocock - it's a slab-winged PA28-180 and I'd almost certainly do it one-up and with modest fuel, as it's only 15 minutes flying time from my home airfield.)