J S Oils seem to be cheaper than most places, they usually have a stand at the Rufforth AutoJumble or you can ring them direct, the number I have is 07879 633313.
I assume they are still in business, it's a couple of years since i bought from them now.
I always buy direct from the factory shop in Brighouse, whether its their Mobil 1 equivelant, hydraulic oils or what ever I need and they always seem to be reasonably competitively prices. I do tend to buy 25ltrs at a time though.