In reply to Bjartur i Sumarhus:
Most of it is made by Santini, which is Italian and therefore small in every dimension except around the top of the knee, where it is too large (for me anyway). Prendas have a pretty comprehensive size guide, which includes measuring a jersey you have that fits well, and using that for reference against their sizing. Anyway, more Castelli than Assos/StolenGoat/Bioracer/Endura, so a good fitting UK Medium jersey would be a Castelli/Santini/Sportful large or (extra large for shorts if you're taller than 5'10'')