Fails to ignite?

I have a Vokera Maxin 28e combi boiler that has an intermitant problem on igniting.

There is no set pattern to the system failing to ignite. The pressure is ok, the temp is ok but on some instances it just fails to ignite. The fan starts to run as if it is going to ignite but the goes into lock out. It looked as though it was only the central heating side having the problem but I notice its now started doing the same thing on hot water as well. Any idea's

Mank thanks