Thank you all so much for your great advice.
@Nick Thomas - I hadn't realised I'd need to combine the capacities of the main inverter and the inverter on the AC coupled batteries for G98/99 approval. This is also relevant to my question of whether to oversize the inverter; just for preventing those times when the inverter becomes a bottleneck (eg all appliances on, or a future large battery charging at limited off-peak times), it would create a far more complicated application process.
I'll ask the quoting company about EPS with either DC and AC coupled batteries. As you say, maybe marginal benefit, but in the countryside where power cuts do happen, it would be useful to both power a battery from the solar array and power the property from the battery.
I will take on board your SolaX experience and ask for a different brand. Are there any you recommend? GivEnergy seem decent (and the company has the Octopus tie-in), and I've read Fronius are too.
@sharpener - yes, it would be electric ufh. I don't have gas (100% electric), and space is probably too tight for an ashp cylinder. I've been reading the other forum threads with interest.
@JamesPa- After reading your example usage breakdown I think, for my size of solar array, I need to shift my thinking. I'd been viewing battery storage as a mixture between off peak grid storage and topping up with my own (potentially meagre) trickle of solar, and trying to cover a large percentage of my energy usage like that. But actually maybe I need to view any solar production as an added bonus for now and think more widely about timeshifting energy purchase. Battery storage is only one option, and maybe not the go-to option either. I like the idea of storing excess solar in DHW, but I'm already considering removing the immersion cylinder to save space, and replace it with an instantaneous water heater, because space is at a premium. I'm also probably not getting an ashp for the same reason. Instead, I wonder if I could view the electric ufh not as an expensive luxury, but as a thermal store. It's not on-demand heating, so if done correctly, I wonder if it would be possible to heat it from the grid only at the lowest off-peak rates (directly, rather than via a battery), with the aim of maintaining a low comfortable temperature, and then top up the room temperature when needed with a log burner or a boost of more instantaneous heat such as IR panels. This would then time-shift a large percentage of my heating, without having to buy a battery. And if left at a low-ish temperature, I could leave it to sort itself out year-round.
For all other electricity usage, I can then use my own production to give a little helping hand, with any excess sold to the grid; and in the future if desired, I could get a battery to store it instead, with the 5-7kwh mentioned by @sharpener and @JamesPa being an achievable size given that heating would be covered with the off-peak ufh and only requiring a top-up.
I don't know if all of that's technically possible or if I'm being wishful; I will also need to speak to underfloor heating companies to discuss minimum insulation and concrete thickness etc. Also, I'd have to ensure it was possible to tie all this into a Hive-style app. Each item I read about comes with its own app, but it wouldn't be great having five different apps on your phone independently running different pieces of the puzzle.
One other thought lurking at the back of my mind is that all the timeshifting of energy usage is entirely dependent on the electricity companies offering decent off peak rates, and it's in their power to take it away if it suits them as more people get electric cars or timeshift their energy purchase.
