Ferroli Modena 80e will not light?

Hello I have read your forum for a while because I have a 10 year old Modena that is good but never trouble free, on delivery it had a small flue restrictor in the top of casing which had to have a service call to find (installer spent 5 minutes looking before calling Ferroli,I have had the dirty fan, the crumbling venturi, slightly erratic APS the thermister problem , that all caused intermittent problems that I was able to fix eventually. I have used pro's before but it has not been a pleasant experience.(1 visit to sevice gas valve on my brand new, gas fire, burnt a hole in my new carpet, condemmed the instalation calling Transco out to sort it, Transco turned up after 20 minutes and "fixed" the main gas tap in 1 minute flat, bless him, so I know there are some good guys out there somewhere.
A few months ago I started to get the occasional lockout on DHW, not a major problem, but read that increasing the lowest Gas pressure 1/2 turn was sometimes benifical, and it did help, no problen since Xmas, until yesterday when erverything stopped.
Now I have a problem that I cannot decide on how to fix it. Boiler calls for power (CH or DHW) pump runs, fan runs APS works , igniter sparks, a very tiny flicker of flame then lockout.
My manual seems to indicate either Gas valve or ignition PCB(MF02)my own feeling is that it is the Gas valve, but I don't know how to test it.
Could anyone please suggest how, if it is either of these items, I can tell which, because they are both quite expensive. This model uses the MF02 boards with seperate ignition PCB.
I would welcome any input, I know my limitations.
Any help appreciated in advance.