Ha found the problem, it was not the valve, but a spoke nipple had pieced the tube. Rim tapes, this wheel still had the woven rim tape on it, but it had dislodged and exposed the nipple to the tube.
Now the tyre has not been off in over six months, the tube has not been deflated, nothing has changed apart from periodic top ups to the same pressure and when it happened the bike was upside down as I was working on the cantis. I took a coffee break, came on here and that was when it happened, for no reason that I can think of other than atmospheric pressure changes, no barometer, so can't check if it was that, but why now, why not out on the trail, as the bike has been pounding earth every day this week and all last and week before etc.
But anyway, drag out the spare tubes, they all have punctures, what's going on, oh and my patches refuse to come off their backing. So a bad day.
But I had one tube left, but it had a presta valve and my rims are for schrader valves, so I cut up a bit of aluminium pipe, threaded it on the valve and fitted, The pressure stayed in the tyre and I am at least mobile for now, but new patches and glue required, everything has perished.
At least the tyre came off the rim without destroying more spoons, oh I love tyres one can pull off with one's fingers.