vokera option?

Hi
I have a vokera option and have in the last 2 years changed the heat exchanger and 2 prvs
it started leaking from the overflow again so i paid for a new prv to be fitted but it still leaks, drips most of the time but when radiators on it pours out
the gas guy thinks i might need a expansion vessel?
question is is he right? and even if he is should i just get a new boiler rather than waste more money
would it be easier to fit another vokera in terms of pipework that is currently their, or should i change to a baxi etc
thanks