An update. I'm now revisiting this, with a view to building and fitting a G100 export limit device, as the DNO have come back with a request that I not exceed 6kW export. Back in 2013 when I made a G59 application (at the time I didn't know how big a PV system we could fit) they originally permitted me to export up to 10kW. When I notified them that I was fitting another PV system, in addition to our battery system (which is set to zero export) they've come back with a request that we not exceed 6kW export, if this is possible. They've said on the phone that their concern relates to several other PV systems having been installed locally since we installed ours.
Have to say the engineering chap I spoke with was very helpful. I asked outright if I could just fit a current sensing relay to the existing PV system AC output and use that to isolate the second PV system AC output if the power exceeded 6kW. He was fine with that solution, said all they needed was assurance that the export would be limited.
The simplest way to do this seems to be to fit another current transformer to the big PV system AC output, and use that to switch a double pole contactor to isolate the second PV system from the grid. Seems this may be easier than I thought, as I've just found these cheap current sensors:
https://www.ebay.co.uk/itm/304631567636 They are adjustable from 2A to 30A, and seem ideal, as I can set it to about 26A and it will trigger a contactor directly.
These are self-powered, from the current flowing through the sensed wire, so can safely be tucked into the consumer unit, with the line conductor from both PV inverters running through the hole. I can run a two core cable out to a contactor box, probably with a manual isolator switch as well (just to be able to isolate from indoors as well as outdoors). The way this would work would be that the power to the contactor coil would be switched via this sensor, so if the combined PV systems generate more than 6kW then the contactor would operate and isolate the second PV system to drop the export to no more than 6kW.
Looking at the specs, the switching capability seems fine for a contactor coil (it's rated at 240V AC, up to 500mA), and the switching time is well within the G100 limit, the spec says within 200ms (G100 is within 5s, I believe). I can easily make it "anti-tamper" by just fitting a seal over the current adjustment after it's been set.
It's not perfect, as it won't be measuring export, as such, but that's only because this cheap sensor isn't directional and in our case any export measurement needs to be at the meter kiosk, which is outside and a fair way from where the PV systems are connected. Worst case is that we lose a bit of generation from the second PV system in the event that we are self-consuming a lot at the same time as the main system is generating 6kW.
We so rarely hit the main PV system 6kW inverter limit, and then only ever in the early afternoon when the sun's square on to the roof, that I doubt this is ever going to be a real limitation. By early afternoon generation from the second PV system (which faces more or less east) will be low, anyway, so turning it off once in a while won't have a real impact.
I'll update this thread with how I get on. If it works then it might be worth putting together some proper instructions so others can do something similar if asked to limit export when fitting a second system.
25 off 250W Perlight solar panels, installed 2014, with a 6kW PowerOne inverter, about 6,000kWh/year generated
6 off Pylontech US3000C batteries, with a Sofar ME3000SP inverter