I've just realised why my gentians are struggling - I didn't realise they were acid loving! So I have transplanted them from one pot to another.
I don't have any ericacious compost though, so it's the normal multi purpose compost again, but I found some ericacious plant feed in the shed and have given them an acid feed. If I do this regularly should it be enough, or do I really need to go and buy a bag of ericacious compost? Would rather not because I don't have any other acid loving plants.