Thermal stores are not rocket science.
Pity so many in the industry seem to think they are. Thinking about it, I'd suggest that anyone who can't deal with a thermal store, or even a heat bank, isn't up to the complexity of dealing with a combi boiler which generally has more moving parts and far more complicated controls.
I think the question the OP needs to ask himself is how much he wants to invest in something that will never recoup his investment.
Sometimes it's not about pure financial payback. Most of what we do to our houses will never recoup the investment - and if you took that line then an awful lot of stuff just wouldn't happen. I know heating is slightly different, but does anyone ever recoup their investment in the big plasma screen, or surround sound, or the nice wallpaper, or the printed concrete drive, or ... ? The answer is no, but people pay for them because it's to make their lives
nicer.
Personally, I'm prepared to "invest" in not having a combi with all it's faults - I know that's heresy to some people, but I dislike them and their fundamental features.
As to the debate here, I'd suggest it sounds like a situation where a heat bank or thermal store would be ideal. Note that I'm only saying it sounds like it would, since details are notable by their absence.
A wood burner gives a nice cosy feel to a living room - and heating water is perhaps a secondary consideration. If you happen to have a good supply of free or cheap wood then that's "almost" free heat, and if you are inclined to count your output, then is low CO2*.
If you have a store in to support he WBS, then adding solar thermal needn't be expensive (which of course is a relative term). No, I haven't done any actual figures on that - yet.
For me there is a secondary consideration - whether this applies to many people is a different matter. The internals of a gas appliance are out of bounds by law for my rented properties - that means I am at the mercy of the people I get in to sort it. I'm sure there are some very good people with diagnostics skills - sadly they aren't employed by the outfit I have a contract with locally (last time I ended up diagnosing the fault myself anyway and telling them what to replace !). And for the avoidance of doubt, this outfit is actually one with a fairly good reputation locally !
In fact I can't even just get someone in as required, the law says (last time I looked) I have to have a service contract for the boiler.
Once I'm past the water valves then it's a different matter. I have the option to get someone in, or do it myself. When a tenant contacts you late on a Friday afternoon that can be quite a factor to consider. Since I only have a couple of properties to deal with, I can be intimate with the way the systems work (and in particular how they work when they are working). So I don't have to spend time finding where the controls are (and even what controls are fitted), guessing at what it should be doing, and so on.
And I can avoid having to deal with someone who's going to start with "<suck> you need to rip that out and replace it with a combi Mate"
* The debate on that is a whole thread of it's own.