Thanks, great reply
I will sort the descaler and inhibitor out.
I switched it back to pump speed 3 and i'm pretty convinced now that is where it needs to be, despite the TRV whistle, air and noise. It fired up earlier on 2, ran for about 20mins and cut out when hot. Then began cycling. Immediately switching the pump to 3 it came back on and stayed on until wall stat clicked off.
My only issue now is the air it creates on 3. I think the water is moving so fast it hits the impeller and creates air. My AAV doesnt seem effective so along with sorting the inhibitor i will ask my gas man to fit a new AAV too, maybe it will help purge the air out.