Worcester Bosch Heatslave 25/32 Combi Oil?

Worcester Bosch Heatslave 25/32 Combi Riello Burner, First off we bought this boiler about a year ago and moved to France thinking everything will work well. To then find out the normal heating fuel in france is 35 sec (red diesel) obviously this boiler wasn't meant to run on this fuel (meant to run on 28sec) but it did and worked perfectly for around 5 months. It then started to play up so took it apart and had noticed it had a large build up of black crap in it. It was cleaned out and it started working fine again. I then managed to find out that all it needed was the jet changing and the fuel pressure and air pressure setting up correctly and the boiler should work fine on the 35sec fuel, which we did. That had worked perfectly for another 5 months then woke up this morning and it had locked its self out. Had it apart again today, no black crap in it insides seem fine. checked photocell that seems to be working, checked ignition thats working, can't see a problem with fuel as bled the system and it's getting to the jet as there is fuel sitting in the combustion chamber. I have put it all back together and tried it again but all it does it lock out after 12 secs, with no real attempt to ignite. any suggestions would be greatly appreciated!?