I've followed your advice added a third filter, moved the co2 timer forward to maximise co2 at lights on and adjusted the spraybars to maximise flow so I hope it isn't co2/flow related. Can't increase the amount of co2 going in without risking CO2 toxicity.
Been thinking about it and the GSA did appear after we had the lights on much longer than usual for two consecutive nights (for maintenance and while guests were here.). I think I'll shift everything back a few hours so that the tank is lit much later in the day to stop these extended photoperiods
The anubias is in line with the spraybar that releases co2 into the tank and the upper spraybars keep the bubbles down well so that the tank is misty with bubbles so think I can rule co2/flow out for now. It isn't shaded so maybe it's getting too much light especially in the past week. Also, the tank is still new plus I added a few more fish last weekend so there may be a bit of ammonia around to encourage the algae.
I'll start with
1 less light (no more extended photoperiods)
2 more water changes to remove any ammonia from the extra fish
3 Replace the eheim air stone with a glass difusser when it arrives from HK to reduce CO2 bubble size going into the filter. The co2 does collect in the spraybar over time. Micro bubbles going into the filter might help more of it dissolve.