I always find what Linksyokel posts on compost very informative.
Seems true to me - the brands are totally variable year on year; for example the New Horizon described is sounds nothing like the bags I purchased two years ago (never again) and the B&Q multi this year is not the same as last year's and their award winning seed/potting compost seemed to cling on to water like nothing ever before and most of my beans and squashes rotted away.
Anyway, I decided to splash out on large quantities of something more expensive and I've been very pleased with the results. If you buy in quantity you get a good reduction and it ends up costing not much more than nipping down to B&Q every few weeks where it seems to be pot luck whether you get a good bag or not.