Thanks guys. You generously offer a treasure trove of information.
I had read about the tapping method in a book (in K.N. Harris maybe...), but didn't realise that it might apply to valves bought from a supplier as well as home made ones. Also, I had forgotten about it
Putting it all together, I will consider the methods suggested for seating the ball, then try one of them. I'll also order a couple of shutoff valves asap - might as well make this as easy as I can on myself
I will also look at the thread Kim suggested, and steal some blue paper from the dispenser in our campervan.
Following which I suppose I had better do another final water test with the valve business sorted just to be absolutely sure the boiler is ok (though I'm sure it would have passed the blue paper test last night) before going on to steam.
And most certainly I shall refrain from testing it in the sun.
gary