Typical of this place in that the main water stopcock in the cellar is a 28mm tap, and I've been unable to find anywhere stocking 28mm taps. I can buy 27mm anywhere, which is no help whatsoever.
By tap, I mean the tap head, the working part. Yes I could perhaps replace the entire tap, but as that is screwed into fairly rusty steel pipework, I could envision serious difficulty and likely damage by trying to change the entire assembly.
Tap heads seem to be readily available, so can anyone recommend a firm, preferably not too far from me...I'm in Chalandray, western Vienne, between Parthenay & Poitiers.

You could try SOS Chauffage in Parthenay

http://www.sos-chauffage79c.com/

my husband gets all his plumbing bits from them... not just heating-related stuff.

Map:
http://tinyurl.com/d5bcgky

Thanks, very kind, map and all!

Went there this afternoon, nice guy but unfortunately no joy, they stock 27mm (equivalent to English 3/4"! wot a larf!) like everyone else, but no 28mm.

A neighbour suggested the garage at Vasles...so went there. Looked really unlikely, all cars, no plumbing. Asked proprietor...suspect he's used to this...& he told me it's the place behind the petrol pumps on the main road out. So soon found it, Avia pumps, with a small brico place behind. Looked pretty unpromising. Checked their plumbing section. It was fairly small, not many tap heads, & no 28mm. Anyway, asked the proprietor; he came round & instead of going over the stock I'd already checked, pulled out an old metal box from under the shelving.....& there they were....more tap heads, including the 28mm one I needed. He was really helpful, took a good while making sure I had just the right fibre washer for it as well.

Fitted it already, no leaks, & so nice to at least sort at last one of the things that's been faulty since we bough the place.