Hi Fred
I'm afraid any answer you receive from this will be pure speculation, as a problem like this really requires thorough inspection to ascertain the cause, that said ill try to point you in the right direction.
With the amount of heavy rain and high winds over the past month or so I've had a few friends asking me similar questions about damp patches appearing with no apparent cause, the driving rain can find its way into a lot of normally water tight places, in one case recently i could come to no other conclusion that the rain was literally being driven through the bricks themselves and bridging the cavity.
As for your situation, I'd first check the roof outside, just from the ground with some binoculars or a digital camera with a good zoom if you can get into a position to see it (or if you have a tall enough ladder) to see if there are any noticeable problems, cracked or missing tiles, cracked pointing on the ridge tiles, also check if any form of vents - extractor, soil pipe, or vent tiles that are next to or above where the damp is showing inside. Any of these pipes or vents require cutting holes through the membrane and if you have missing or cracked tiles or even if the driving rain is being pushed under the tiles, then that COULD be the cause.
With loft conversions vent tiles should be installed to keep an airflow through the roof void, lack of these vents COULD be causing condensation building up on the underside of the membrane,
Secondly if you have access to the loft space then take a look around for holes in the membrane, staining or discoloring of the timber rafters is often a big give away, and you can literally follow the trail to where the water is getting in.
If none of those point to the cause of the problem, the some exploratory surgery is required. If the damp inside is bad enough that it's not going to dry it out on its own or damaged the ceiling then remove some of the affected plasterboard to see if you can trace the problem from inside, failing that the only course of action is to start stripping the tiles back until you find the problem.
I'm sorry i couldn't give you any definitive answer but i hope i have given you enough to be able to track the problem down, Best of luck and i hope the rain holds off.
Dan
