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.
It actually gets easier as the week goes on, as the number of US-centric questions goes down…