So, first morning of the long weekend.
Have you noticed that wet patch says she!
Fortunately near an existing access point in the landing floor. A 15mm copper pipe with a slight weep although her vanity mirror got lost under a Gorilla's armpit of pipework. Bathroom soil, shower waste, basin waste, 6 lots of 22mm pairs... heating, boiler flow/ret. and hot/cold offtake. So, through the ceiling then. Nice rectangular cut for easy re-fitting. Levered it down carefully and got an immediate shower of water, about as fast as I used to be able to piddle!
Not mains, turned off. Not gravity cold, turned off. Not hot.... oh s*it it must be central heating. But the valve is off, the rad stats should be closed... oh s*it again it must be the bathrooms which are open for boiler overrun!
Luckily we have buckets and the water was surprisingly clean.
The fault turned out to be a galvanised nail (plasterer) which had just entered the pipe wall leaving a neat square hole. Nicely sealed for 23 years so why pick today?