Thank you - yes, I’ve really struggled here - despite the pot feeling light and the top layer being dry, it turned out the lower levels were soaking. I’ve been (annoyingly, according to my wife :-) ) very careful to move the plants out of shade though, so hopefully that part isn’t a problem.
Thank you also for the very detailed reply - from those options I think 1) and 2) are unlikely in my case - it’s a pre-fertilised compost designed for cannabis and should still have enough nutrients (plus during various stages of panicking I added N-Rich fertiliser (Fish mix) and no change occurred). This leaves 3, which has been the suggestion of others as well - although from your explanation I’ll be careful about reusing compost and will use new compost and add further fertiliser if required.