I've been using b&q's multipurpose off and on for years and I think I've only ever had one bad batch (and that was just a bit coarse, there wasn't plastic or anything in it). Maybe I've been lucky. I had some really rubbish stuff from Morrisons last year though - it ended up in the bottom parts of large pots that I put my tulips etc in last autumn, and the rest as mulch. It was very cheap, but I've had very cheap stuff from Lidl which was fine. I suspect the common factor is that the cheaper ones are more variable (maybe from different sources at different times) whereas the more expensive ones have a more standard recipe.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
Posts
https://www.which.co.uk/reviews/compost/article/all-compost-test-results