Well.......... (ahem)( you were waiting for this!)
I left the stat switched on and came downstairs to do other stuff pricipally helping W make tomato soup, but also to make a coupla phone calls.
W had put a pan on a gas and I said to her, what's been burned or scorched? so we looked around couldn't see nuffin, so I went upstairs to check and, you guessed it, there was a faint trickle of smoke coming out of the back of the stat!
I moved the switch to "off" no diff, indicator lights were flickering, The original fault, so I went downstairs and switched it off at the mother board.
So on further investigation, the relay is toast, the new capacitor ditto, (it had been touched by molten stuff coming from the relay which is right next to it.
So I took the opportunity to nip off the two other capacitors which others had said could be dodgy, measured them and both were fine.
So, was the relay dodgy anyway, or had I been hamfisted with the soldering?
One of the six terminals to the relay is indirectly linked to the path that comes extremely close to one of the terminals of the capacitor. I have to imagine that it was this, or the relay was on its way out anyway.
I think I am going to have to modify my soldering iron, i.e. to grind the tip down so it is much narrower.
I think this was only to be expected from an amateur.
Doubtless I'll get better with practice.
Ive got a gas soldering iron so much better to use.