If it's a diesel or a 2.0l V6, I can't help you and don't bother to read on.
Presuming it is a K-Series engine (1.4, 1.6, 1.8):-
If it is definitely the head gasket, I would suggest fixing it now, otherwise it is likely to overheat and you'll end up needing the head skimming. If you are able to do it yourself, it is not a particularly difficult job. You might want to consider doing a cam belt and water pump change at the same time (unless you know these have already been done recently).
I don't know the 45 particularly but you may need to check the liner heights to determine which head gasket is the best one to use (there are a few options). It is also usually advised that new head bolts are used but not everyone adheres to that.
You might find it useful to look here
https://www.dmgrs.co.uk/pages/how-2-gui ... r-mg-rover.
If you need a garage to do it I would expect them to charge somewhere in the region of £400-£500.
It is possible the problem is the inlet manifold gasket, which is cheap and easy.
A more knowledgeable member may be along soon to offer more specific advice. If not, I'll try to answer any further questions you may have. There will be previous posts on this forum relating to this issue. You could try searching for/through them.