This is a really interesting discussion. Firstly it displays the ignorance on offer in some garden centres about weedkiller residues entering the 'compost chain'. The most likely culprit is clopyralid, a sibling of aminopyralid. Clopyralid is a persistent lawn weedkiller that can cause damage in tiny amounts. It gets into bagged composts via green waste recycling. It can crop up in both peat-based AND peat-free composts, because most peat-containing composts are now peat reduced, and the reduction is often achieved by adding composted green waste (which is done to high standards).
The second thing some of the comments show is how misleading scaremongering in some national newspapers has seeped down into the minds of gardeners. This is a serious problem of pollution, but some commentators have chosen to use it as a way of attacking peat-free composts rather than alert gardeners to the pollution being caused by clopyralid weedkiller.
The following links might prove informative:
1 My 2012 trial of almost 40 peat-free composts which showed that apart from a small number of exceptions, peat-free mixes can produce great results.
2 My article on clopyralid weedkiller which explains why it's a growing problem and what we need to do about it.
3 My comment on the dangers of sloppy and misleading gardening journalism.
Gardening & environment writer, author