I have had the same problem, the core plugs get eaten away from the inside, they look fine on the outside until the leak appears, my boiler was only a few years old when this happened!
The core plugs are identical to a ford transit and are readily available from any motor factor, I replaced mine recently, to do this you have to remove the boiler as the heat exchanger is bolted to the bracket which holds the boiler on the wall and strip the pipes etc to get it out, it is a pain of a job but not that hard to do.
I am currently trying to source some stainless steel core plugs to install in the summer when I can take more time to do the job without worrying about the boiler being out of commisin for too long.
with the stainless plugs installed the boiler should be good for 20 years.
I can't imagine why potterton fit these thin plugs on a boiler that would otherwise last for years.
Robin
October 2009