The new suprima board sparks for 5 seconds (even after flame detection). My best guess here is that the ignition lead is broken.A small break will allow the spark to jump it, but not allow flame sensing. To test - continuity test the lead.
I do realise that this has been posted 10 years ago, but your post helped me to fix my boiler in 5 minutes and I'm office worker, not a gas man. I have removed boiler front cover and found broken ignition lead. Everything is working now. I have signed up only to say "thank you so much" .
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