TY all. Really helpful.
So G98 notification to my DNO is still required for a non-exporting installation ?
On the whole meter backwards thing, I'm sure my parents meter does so (10+y old MCS install). I need to check if they were mistaken....
JohnMo mentioned "With a 2kW array and I assume an immersion diverter, you would be very unlikely to export, except for a few weeks per year if no-one is home during the day." - thats just the issue, even with a small system, most generation will be during sunlight hours and most usage outside of this, so exporting would still happen (well, only when immersion is up to target temp.). Can mitigate this somewhat by running some high loads during the day but I also have a lot of kit that runs 24/7.
Have done plenty of number crunching, and even this £1800 2.4kW system will have an ROI of 1.8 years (at expected prices coming in October). But of course thats presuming 100% solar usage.
Immersion diverters are fine, but not that efficient as all you are saving on is the gas you would have used to heat the water, and gas is way cheaper for heating things.
So really its either this solution or going the storage route, but then the battery investment is considerable and will ruin the ROI further. Its such a shame - we already have access to a giant battery - the National Grid.
I wouldn't be comfortable waiting longer than 2 years for ROI - this is primarily a cost saving exercise for me personally.