Failing hot water on a Saunier Duval F30E Themaclassic combi?

I have a 4 yr old Saunier F30E standard combi running on natural gas and the hot water is failing dismally. However heating works AOK.

Symptoms-

turn on hot water, boiler fires, flame spreads across whole burner, burns for about 10 seconds, goes out, 15 or so secs later flame re-lights, REPEAT

Items checked so far-

cleaned boiler as per installation manual.

set sensor electrode and ignition electrode gaps to 3.5mm as per manual.

measured voltage at H2 from flame sensor electrode, result 6 volts when flame covering electrode.

disconnected flame sensor, result, lockout with fault code F1 shown on display.

measured voltage across all 3 leads of flow sensor. result 6-7 volts across each terminal.

Checked Control board PCB, no sign of dry joints on soldering. resistors appear to measure OK.

BTW, I am a sparky, and have serviced this boiler regularly plus flushed system and used inhibitor, 18 months ago replaced three way valve actuator and diaphragm, both still working and A1 condition.

Any Saunier experts with ideas of a fix, (don't much fancy paying £150 for new PCB)

TYIA