Thank you all for your input.
I have narrowed my choice down to 2:
Worcester Greenstar 40 CDI : £1077, 16.3 flow, 30Kw, 2yr warranty.
Vaillant EcoTec Plus 837 : £863.11, 15.2 flow, 28Kw, 2yr warranty.
I will double check the flowrate of my mains this evening.
The reason for high...