Ravenheat CSI primary not igniting, can you help?

I have a Ravenheat CSI primary condensing boiler, this is about five years old and has worked okay.
We recently went on holiday and switched the controls off for both water and heating ( as we normally do).
When we switched back on, both water and heating call for heat, the pump activates and the boiler fan kicks in, however the ignition light illuminates for a few seconds, then goes out and the peizo ignition can be heard clicking away, but there does not appear to be any gas flow.
It is as if some gas valve has seized up whilst not in use. Any idea's please.

Joules
August 2008