Potterton Netaheat 16/22 - rusty backplate?

Hi there -

can anyone help me with this please? We've just had our boiler repaired and the guy said the backplate was rusty on one side - he checked and said it did not have any holes, but that we would need a new boiler soon. When I asked about just replacing the backplate, he seemed to think that was not a good idea. The boiler works fine and I can't afford a new one - assuming we can get the right spare part from Potterton, would it be difficult/expensive to replace the backplate? Would it mean taking the whole thing apart?