Sorry, I forgot you might be hooked on my toilet trouble drama and thusly I owe an update.
It seems to be fixed. No more puddle (leastways until the FiL visits, he must use some kind of Hozelock sprinkler attachment).
It didn't do too much damage (there's a wooden floor in the downstairs loo, not sure why we didn't have it tiled, anyway) other than ripping out all the panelling (mostly held in place by mastic). Yes, yes, Mr Grown-up, it surely would have been a better idea screwing the panels in place. Don't complain too much, we'll be paying you to put it all back. He also pulled out the disco-LED from the bathroom which, in the spirit of easy replaceability, involved yanking out a piece of ceiling to extract the entire unit. I think he just likes smashing bits of my house.
Turned out to be a cracked attachment where the cistern pipe joins the toilet proper. The chap who fitted it must have known it was cracked as instead of getting a new one, he's just put mastic all over it. Not sure why it took five years to decide to leak, but there you go. It happens to us all in the end.