Potterton Netaheat 10-16 Mk 1 boiler fan problem ?

I have a Potterton Netaheat 10-16 Mk 1 boiler.
It has recently started having problems firing up. These are intermittent. The boiler thermostat is on and usually I then hear the fan starting up and then the pilot ignites and then the main burner ignites.

Sometimes when the boiler is turned on at the thermostat nothing happens in that the fan does not run and so on...

However, if I give the boiler casing a sharp tap near the small viewing window, the fan will then start and everything will go as normal.

Could this be a fault with the fan? If so, I know I can get a replacement fan for my boiler. Is it a replacement job I can do myself? i.e. remove the boiler case and take off the old fan and fit the new fan? I am competant in electrics.

Or is this a job that will definitely need a gas serviceman to do?

Many thanks.