Drywsdad............I have used B&Q's Verve compost in the past............not exactly wonderful stuff I admit and I prefer other brands.
However, I did buy a couple of trays of Snowdrops earlier this year......they had more or less finished flowering but were still "In the green" and they had been reduced in price.
When I got around to decanting them from the tray, I found a couple of cells with nothing in so took that tray back the next day. Not only did I get another tray but also got some money knocked off because I had, in their words, "had to make a special trip to return them ".
If, when you open your compost bag and find it is not as you think was described, I'd certainly try taking it back and explaining why.
It could be that complaints are dealt with according to the Store Manager of each particular branch but if you are really unhappy, I'd say it was worth a go. If enough people complain, even these mega chains eventually take notice