Thanks so much guys for all your help and advice, I am literally close to tears here just thinking about it.
My plan of attack is:
Let him stay nappy free tonight,
check him every couple of hours at least and change any wet prefolds for dry ones,
keep checking if he's warm enough (he has on a short sleeved and long sleeved t shirt, and 2 fleece blankets, but the room is only showing as 16 degrees at the mo
see how he is in the morning and if it's no better take him to the docs (I just read in the nappy rash section a post about someone (AnnetteandBen maybe?) who's lo was getting really bad rash and it turned out his zinc levels may be low due to prematurity which makes them susceptable (sp?) to more infections etc so i will enquire about this too as Archie was 8 weeks early.
FC it looks better in the morning, well it couldnt be much worse