Cheers Val. I have found this site, but in order to compare prices you have to price up every possible permutation for each and every crossing in order to compare them. This is very tedious and sometimes just tells you that they are unable to quote for that particular crossing when you've spent ages filling in all the boxes.
The best price I have found so far is with MOBY ferries which is £165-ish for car+2 and a cabin (overnight) I think that's probably about as good as it's going to get.
Thanks for your suggestion.