They've just rung up to say they can't do it as "the heat exchanger has gone". I'm hopelessly non-technical so have no idea what this means or entails, other than it being the cause of an MOT failure and apparently its insanely expensive to repair

Can anyone advise please? What should I do? I live in Hants - can anyone recommend a place locally that can sort this out? And has anyone got any experience of having this done before and if so, just how expensive was it?
*fed up*
Stimpy