the widely held belief that the panacea to all aged boiler woes, is to renew with one of these fiendishly sophisticated modern units.
No ones sugesting that is the case and I regularly tell customers to keep hold of boilers which I believe is uneconomical to replace.
Which if truth be known,on the savings you are likely to make on gas, you are very unlikely to recover your initial investment, before you have to renew again.
That depends on many factors, one of them the installer. I've seen many new boilers fitted which are not operating efficiently because the installers lack of knowlede or care. The boiler is just a tool for an efficient system and many old boilers can be made to work more efficiently with alterations to the system. Some installations you are correct though, you wont get it back.
And that only after spending £lots on repairing these complicated electronically controlled boilers.
Maybe if you buy an unreliable turd. Again you need the right installer to recommend you a reliable product. Its no more complicated then your car and I don't suppose you expect that to keep going wrong all the time and last 5 years. Many condensing boilers will give many years of reliable service. Others will not!