Smells like it might be a bad electrical connection either in the general wiring or on the control board (pcb).
Next time if fails you might consider taking the lower front covers off and gently tapping around with a wooden spoon to narrow down the exact place that needs the bashing.
Here's a link to someone repairing a Potterton pcb - relatively easy for a person confident with a soldering iron.
http://kaijaks.co.uk/blog/nick/2008/02/10/dodgy-old-boiler
Ignore any well meaning person who warns you about the legality of self repair on gas appliances - this link will explain the legal position for a householder...
http://en.wikipedia.org/wiki/Gas_Safety_(Installation_and_Use)_Regulations_1998
i.e. You must be Registered to take payment but just competent if it's your own boiler - competence is not defined in the regulations.
Good luck...
Peccavi
December 2009