I've turned off all three valves flowing from the tank in loft in turn and none seem to isolate the bathroom cold water.?

As you just demonstrated you DON'T know which valves control water to which parts of the home, the ONLY useful advice is to have someone LOOK at your plumbing.

It SOUNDS like you are shutting of the water coming OUT of the water heater, NOT the actual main shutoff. Without being able to actually SEE the pipes, a guess is the BEST you can hope for.

sounds like you just turned off random valves and hoped for the best. there should be one by your incoming cold line to your tank. there should be one by your main(usually where your water meter is) and one right after the tank. you most likely just turned off the hot water from your tank. if your having troubles figuring out which is which one you should just shut the main valve off and that'll shut down water to your house. if it doesnt have one and you have already called a plumber to help the last resort is call the city and they can turn it off from the street but they are usually reluctant to do that because that may affect everyone on the block potentially.

Even after you turn off the stopcock, you can get a few pints of water out of the cold taps. Even if the cold tap is mains fed.

Try that first.