Fuelsaver 55F not igniting?

I have a Fuelsaver 55F problem, when either HW or Heating is requested the pump in the airing cupboard starts, the fan on the boiler starts but the boiler doesn't ignite unless.....you slap the boiler casing (not hard) and then it ignites!? It won't keep running though and the customer is currently setting an alarm to go off every ten mins so they can get up and encourage the boiler to perform. The gas safety guy helping me sort this thinks it might be the pressure switch but we haven't been able to locate it yet? Customer doesn't have manual but I've got one coming. Can anybody help?