The ensuite bathroom is located directly above and the water was coming from there.
I turned the water off at the mains and turned all the tapos on to drain the system but it was still pouring out.We called out for an emergency plumber who turned up 30 mins later. It was a good deal, we were told, because there was no call out fee. They only charged an hourly rate of £119 per hour!!
The plumber told us it was a simple job and could be done within the hour. It ended up almost going into a third hour by the time he finished! So after VAT that came to almost £300 which I thought I could claim back from the insurers but now it turns out I can't.
Anyway it turns out that it all happened because we've had really hot weather and the plastic brackets that hold the concealed cistern in place had warped and the cistern came loose and emptied itself through our bedroom ceiling.
The original plumbers that fitted it should have used brass fittings but took a shortcut and used plastic instead. Thanks guys. It'll be weeks before we're able to use that room again. Life's good ain't it?!!