Go for a combi, any changes won't be retrospective. There is lots of life left in the gas option for now.
Yeh, I think, based on some rough numbers, that it makes more sense.
If I replace the cylinder with a unvented direct, I am essentially committing to an all electric set-up. This would mean the perfectly good manual storage heaters would need to be updated to HHR at some point (even just for EPC purposes). The new storage heaters would unlikely (in real use) be any more cost effective than the old ones. They would just have a few more bells and whistles to make them more user friendly.
So:
Cylinder replacement £1200 (approx)
3 HHR Storage heaters £2500
Modern storage heaters require a permanent live and i don't think I have those in close proximity so there would be extra costs there too.
So if we rounded it up to perhaps £4000 for the electric route.
Which in reality would just give me a modern version of what I have with only a minimal increase in efficiency (in terms of bills), slightly better hot water pressure and a slightly better EPC.
My ballpark guess for a new combi is and all additional work is about £5000.
Much better EPC.
Lower energy cost to whoever lives there.
More user friendly
Increase in flat value that offsets the cost? (spurious one perhaps)
Downside is ongoing cost of servicing.
But I think the upsides and cost difference swing it in the combi direction when looking at the cost difference alone.
Unless I am way off the mark in my ballpark guess.