MAIN 24 he combi

Hi
my BAXI / MAIN 24HE combi boiler is doing a unusual flame failure issue
after it had the normal leaking hot water on demand diaphragm leaking , so i replaced the complete valve and bought a spare seal kit,, so all replaced all good , (no gas valves or disconnect etc involved)
i am a electrical and mechanical engineer for over 35 years, and i have done my pat testing etc and 18th ed 12 years ago, so iam ok with a fluke lol,
the hot water on demand fires up the pin micro switch all good then fan kicks in then ignites the gas immediately , runs all good nice and hot clean burn ( cleaned out injectors and gas / air bed nice constant hot water on tap and central heating side etc

then when you turn the tap off or when the heating goes off then when the boiler runs down and stops ,then the flame failure light starts flashing , i do have a new board i can flick over , just wanted some one who maybe has experienced this issue causing this flame failure after shutting down ? by the way i have changed the thermo over heat sensor the left vertical pipe just about the temp sensor, water trap all good, the compartment furthest away from you is 3/4 full water the compartment nearest you ( evacuation pipe) thats empty empty
any advice would help,

thanks