Otherwise it's easy enough to fit an OFGEM approved total generation meter to the off-grid inverter output. I might just get one and apply for the RO and see what happens in April...

Thats what I have done. I can also compare the power the MPPT says it made with the power through the mains meter & see what effect the charging losses are. (which I dont think I should get paid for as its a simple system loss just like controller & inverter inefficiency losses)
I have had to change how the power from the genny is distributed, so it now goes via its own consumer unit & ring main as a totally separate system.
(we had this in place as heavy loads have there own sockets direct from the genny so was simple to extend it)
It also means I need to monitor the weather & the battery condition to better match load & supply. I will also be fitting a second bat bank & inverter for use via genny based charging. That way in winter I can leave the solar bank fully charged & use the genny bank.
With the increase in our array size I think that we should be 100% solar from 1st March to 30th September ish, 40% in Feb & Oct, 15% in Jan & Nov & 10% in Dec.