Whats involved in replacing back boiler?

If practical you should aim for an efficient combi condensing boiler to replace the back boiler. They are neat and can be fitted in a variety of places. You can then say goodbye to the gas fire and the hot water tank. The existing pipework and radiators should be ok to use. It's just a matter of connecting the new boiler into the old system. The hot water pipework would need altering as the new combi boiler would supply hot water at the taps on demand rather than via the tank as before. You need to get at least three quotations (not estimates) for the work. Endeavour to compare like with like so you can judge the best deal. I would start with British Gas as they will suggest the best options. Ask subsequent firms to quote for the same work. It's tedious getting quotations but at least you will be better informed and at the end of the day you can decide whether or not to proceed or save up as required.