Dripping boiler overflow

We have a Worcester 28CDI which has a constant drip from the overflow outside. We don't top it up but the pressure keeps varying between 2 and 4. Local plumber has replaced valve, but problems persists, now says is the heat exchanger plate that will have pin hole, meaning water from hot water system getting into central heating system- £300 to replace.
Any cheaper suggestions out there?
Thanks

is the filling key removed? if not, remove as it is not supposed to be there any way. apart from when you top up the system that is.

Got a static caravan.

Is it free over the new year.

If so Ill come and repair it F,O,C for the van on newyears