I splashed out and bought a heat exchanger. Quite a big one ..... I don't know it's rating but 30 plates and 32cmm long it should do the job
My first test gave my fantastic results.
And my local boiler spares shop sold me one at ebay Chinese prices!
Mains temp 20.5c Boiler temp 99c
Boiler tap open full gave gave me 22.3c
Boiler tap 3/4 open gave me 20.7c