In reply to rob wmc:
Definitely start with some new shorts.
I wear Assos ones for long rides, especially when warm, and they're fantastic - only had them for a couple of years and alternate 2 pairs. They are an expensive treat, though.
I also have Castelli, Endura and an ancient pair of Nike ones that are pretty tough, but the padding is rubbish and so they're reserved for mountain biking and short rides.
Castelli have a good reputation for affordable quality and DHB (Wiggle's own brand) seem to be pretty popular.
Try:
http://www.sportpursuit.com/road-cycling?type=Shorts,Trousers_%26_Tights