Thanks very much. I've dropped it back to 25 (from 26) and will change it to 24 tomorrow so it's not too sudden for them. Does the 25% water change and Easy-Balance sound like a good plan considering I'm now (potentially) basically performing damage limitation?
ETA: By the way it was the LFS who set the heater and told me not to change it. I see a pattern emerging... :/
Personally if you've got ammonia/nitrites in the water, i'd do a bigger (and more frequent) water changes, probably better than putting more chemicals in the water!