We have a reasonable design I think, certainly not the best but pretty good considering diminishing returns.

Done a lot of work to the house so it's got decent insulation and draught exclusion considering it's vintage.

The main determinant of efficiency (actually COP, coefficient of performance, or SCOP, seasonal COP) is anyway system design and how you operate it.  This can make the difference between a SCOP of 2 and a SCOP of 4 or more.  Best focus on that initially.  The next main influence (albeit on cost not scop) is electricity tariff.

As part of this decision we have been thinking about the spark gap and running costs.

Of course, we don't know the efficiency of our 18 year old gas boiler to understand its contribution.  We can look at it's specifications and see as new it was rated at 88.9% efficiency.  Interestingly we don't know under what conditions that is, and it might vary widely depending on what percentage of duty cycle it's condensing function is used.  But even then, with scaling build up in the heat exchanger, and general wear and tear, it's bound to be less effective than it was out of the factory.

So let's say it's now 80% efficient for arguments sake.  Our new heat pump should be 360% efficient, giving a 4.5 multiplier, and should be cheaper to run, yay!

But how long is that 360% efficient heat pump going to stay at 360%?  What levels of performance degradation are reasonable?  My solar panels came with a guarantee of less than 0.7% performance degradation for 20 years, 10 years in I can't see any difference in output.

The boiler has been maintained well over its life, it even had a new heat exchanger in it's warranty period, so we have to assume good regular maintenance of the ASHP too.

Anyone have long term performance numbers?  Would they be comparable to newer designs?