Cheers to all, apart from advice to hold sellers accountable I've picked up info on best brands, just sieving normal earth and compost, and making my own and incorporating worms in the process... all of which will be useful and I absolutely will try to get a worm compost bin going this year. It does seem like sieving is the sensible first step with all general purpose compost.
Edd, sorry if I posted on a topic that is well provided for - in fact I wrote that post a few weeks ago but when I came back the other day and was prompted to verify my account, this topic was automatically revived.
Welshonion, thanks for clarifying about the peat (incidentally it was an honest question, I don't claim to know!), and I have to admit that with such a gulf in quality the temptation to choose peat and stick with it is very great! But I'm encouraged by the above discussion of alternatives!