I have a pressurised central heating system which should be about 1 barr but continually loses pressure. The house is very old (18century - the heatng is newer though) and so not best insulated. The mansard roof area (bedroom is fairly hot in summer and cool in winter) is Ok but the semi-underground bit which is our living area gets cold in winter and yet the radiators there never really get what I would call hot.
A heating engineer tells me that without putting in a new boiler the loss of pressure cannot be found or resolved. I'm not good at DIY but that seems odd to me.
Any ideas?

Loss of pressure means a leak. Just replacing the boiler will not find the leak so I have no idea what your supposed engineer is talking about and he probably does not either since he only wants to sell you a new boiler.

No matter how old the house is the leak needs to be found. Once that is done and the leak repaired you can move on to finding out how to better heat your home.

On old homes with hidden leaks if you raise and maintain the pressure while doing a inspection of the boiler and pipe in the home. Some areas of pipe might be hidden and here is where a younger man who is more flexible and able will come in handy looking in all those not easily accessible places. Just look for the wet spots to find the leaks.