(just finished writing this once then stupidly went back to page 1 in my browser, and lost it all!)
Colinsk your comment about the plants makes a lot of sense - will watch my pruning in future. Pockets of gas / 'bad' bacteria that has been mentioned by many also seems to be a likely culprit. Note to self: find chopstick and stir gravel tonight!
Have tested the tap water for ammonia / nitrite - zero for both. I match the temperature as best I can for water changes - add boiling water (don't use hot water from tap as pipes are old). Yeah I don't entirely trust Sydney water, but I think in this case its not to blame ...
Over the last day I've been basically managing this nitrite spike - been having to do 50% water changes 2x day to keep my fish happy and nitrite under 0.25 (ammonia is zero). No more deaths, but my poor gbr is the best barometer - he's hovering at the top of the tank a lot of the time but instantly after a water change he's down the bottom in his cave, still looking cranky but not so sick. up and down, up and down ...
I've fed only once in last 2 days - keeping food waste / fish poo to a minimum until things stabilise.
I said in my first post that my pH was fine - but actually it is not! tested yesterday and it had dropped massively to 6.6 (out of tap its 7.4-7.6 as per usual). This has not happened at all before - I'm thinking the black / 'bad' bacteria - if they are producing rotten egg gas that will be the smell - also ... remembering my chemistry - h2s is water soluble and acidic - pH drop explained yes? Therefore can I expect my pH to stabilise once everything settles down again (as long as I stir gravel regularly to stop bad bacteria building up)? I'm hoping too my fish can just cope with the pH swings at the moment - nitrite prob far worse.
2nd note to self: revise aquarium /nitrogen cycle/ bacteria chemistry ...
Thanks again to everyone, the feedback has been really useful.