Aerosol cans

Quite possibly. Try placing the spray can in a bath of warm water for about 15 minutes or so. This has worked really well for some of my projects. Make sure you allow the paint extra time to cure due to the colder temperature. And DO NOT use hot water, unless you want the contents of your spray can all over your house.
Furthermore, make sure the can you're spraying with isn't almost empty, and examine the spray valve (the piece you mash with your finger) to ensure it's not clogged in any way.

Gip Winecoff

Thanks I'll give that a try. One more question, would I have similar issues using an airbrush with a compressor?

The compressor has a moisture trap but I've never used it in a cold environment.

Many thanks for your help.

hiya dan just an idea but if you plan on spending time up in the loft get one of those greenhouse heaters you see round the market or robert dyas cheap to run and warm quite well

Aha!

great idea, I have a small electric heater up there but it's only effective if you are sat on top of it!! I'll have a look around for a greenhouse one.

Cheers.