If you are near Trowbridge then try out www.nutz4bikes.co.uk they have a good selection and price range. Maybe get ones with liners that can be removed, i'm currently wearing trousers and over-trousers with a liner as it's cold, you then have another layer to remove if Spring gets here...

I ended up with XXXL to get the length in the legs as the knee caps rose above my knees after i got comfy on the bike...