Based on discussions in other recent threads, it seems to me that there can seldom be a financial justification for replacing an operational boiler purely to achieve higher efficiencies and lower running costs.
Looking at my own situation (12-year-old Baxi Solo 70/PF) - using the information on the Sedbuk site
Since I currently spend about £400 p.a. on gas, I might hope to save £100 p.a. Assuming a £2k project cost for sourcing and installing a new high-efficiency boiler, it would take about 20 years to break even, by which time I'll be over 80, and the 'new' boiler will be long-since dead, even if I'm not!
Of course, one would normally upgrade the control system at the same time - increasing overall efficiency a bit more in the process. In my case, I have a fully interlocked boiler (Y-Plan) already, so my boiler isn't needlessly keeping itself hot. However, I don't have TRVs, and am conscious of the fact that some rooms are too hot some of the time in order to ensure that others are hot enough.
I suspect that's it's much easier to justify the outlay on tweeking the controls - particularly if you DIY. I reckon I could probably save £50 p.a. for an outlay of about £200 if I were to fit TRVs on most rads whilst keeping the current boiler - and this is a much more pleasing proposition.
Looks like my project for the summer is mapped out!