All sorted now, thanks guys (esp. Mandate!). Can't believe it took me so long though
After managing to loosen it with a long, metric ring spanner (the 7/8" AF one I bought was a tad too tight), I switched off the water to the whole terrace in the road, in the middle of the night to avoid inconveniencing too many people
I couldn't find anything that fitted - my tap reseater didn't, nor did the tops of modern 15mm & 22mm stop valves, nor did the guts from a load of old (1930s-50s) taps I'd ripped off old basins in skips
But with a vice, some good spanners, water pump pliers and WD40, I managed to separate all the bits quite quickly, polish them up with a wire brush and a fine sandpaper block, then re-assemble with a new washer and about 1 yard of PTFE tape rammed into the packing box and squashed down with the nut. I managed to fix the tap head back on with a thin nut and bolt running through a 2.5mm hole drilled through it and the spindle.
Switched the water back on in the road... perfect
No leaks and shuts the supply off 100%.
I'll make sure I operate it a couple of times a year to prevent a repeat performance.
N.A.