The valve has stuck open.
After getting dressed and rummaging in my bits box I found that the thread for a radiator valve is the same as the one for the shower head, so I put the one I had in there on instead of the spray and screwed down the decorators cap. It stopped. Phew, glad this is gravity and pump, not mains :) and so to bed.
Today intend to take it apart to see what's wrong. Start with the isolator valves (none fitted) the tank drain (none fitted) the bungs I don't have to fit, not even a carrot. Or rather two, as we have two tanks in parallel. And I don't really want to run the system dry, that's
100 gallons, and you just _know_ we'll have a shortage soon... anyway we're metered.So tomorrow I need to find some reasonable sized bungs (or a couple of suitable sized root vegetables) so that on New Year's eve I can take the thing apart and see what the problem is. If I can't fix it - well one possible spare is over £100. But I mustn't break it so I can't reseal it and refill the system / pull the cork out :(
Glad this is the en-suite, so we have another :)
Andy