Boiler refuses to fire up?

I am having terrible trouble with a Heatline S30 combi, can anyone help me please.
Yesterday it intermittently ran but now & then would stop & not fire up again, after 3 attempts boiler goes to "lock out: leaving 90 degree light flashing, the book says this indicates flame failure.
after a full strip & service it has now become a permanent fault. Does anyone have any ideas that could help me please