Is this a major problem with the central heating system or one
that I can easily repair?

It is a major problem. Water is clearly getting into the system. You need to
identify where. Some possibilities.

(a) DHW heat exchanger in a combi boiler.
(b) Leaking indirect coil in an unvented hot water cylinder.
(c) Filling loop not disconnected after filling.

Basically, anywhere mains pressure water comes into contact with the heating
primary circuit is suspect. Depending on what the final diagnosis is, fixing
it could be cheap and easy, or very expensive.

Christian.