Question Author
Thank you all for some really great answers and I do so agree with what ladyalex said about insurance companies. There was no damage caused by flooding as I was able to shut off the water supply etc.
The cistern tank has a hairline crack in the base running from the overflow pipe to the 'o' ring in the centre. Now I've checked things out there is a small amount of free movement between the cistern and WC bowl. My theory is that when someone, possibly me, sat on the seat, the bowl caused some leverage of the coupled system, perhaps in the area of the connecting bolts, , creating the crack. This would have the same effect that craftypig suggests - stressful movement.
Anyway, I'm left with no upstairs loo, trouble with the insurers, plumbers to deal with. All very stressful and potentially costly. I'm not flush, and will be mightily relieved when the mess is cleared up !!
Cheers.
D