As I understand it, the Which best-buy advice refers to B&Q composts tested up until 2011. I have always used their multipurpose and had good results from it in the past. I believe they are now selling two types: the stuff in the old purple bags (which I assume is last year's stock) and the new 'Verve' compost.
My mother tells me that she read an article about the poor quality of Verve compost - that it contained lumps of wood, pieces of plastic and other rubbish. I haven't seen the article myself.
I am using purple-bag multipurpose this year and am experiencing variably germination. Ricinus has been fine (tough enough to overcome anything?) but none of my cosmos has germinated and I planted several trays of it.