I had been putting this off for a long time, as in my mind (and from what I'd read online) it was going to be a bit of a nightmare. Also, expensive.
So I'm pleased to report, that my experience in successfully getting G98 approval was incredibly easy, unbelievably quick, and didn't cost a penny.
I found out who my DNO was (National Grid), and found a "Please call me back" type form on their website. I gave name, number, and a very brief outline of what I was hoping to achieve.
12 minutes later I received a call, and a guy called Justin explained how the process worked. He also said that from the information he had about my specific house connection to the grid, it was almost guaranteed I would not get the 8kW inverter approved for connection. He explained that the very best he could do was 7.38kW. While he was on the phone I looked through the ENA website at Victron inverters, and suggested I might downsize to a 6.5kW unit (Multiplus II 48/8000), which he said would be fine. He also went through the G99 form with me, and answered various questions.
I filled this in, and emailed it over to him. This was Monday afternoon last week. Tuesday at 8:44am I received an email with an attached letter "G99_Fast_Track_Approval_Letter"
It was that simple, and it was absolutely free.
Mind. Blown.
The letter states that they'd like to see the system installed and connected within 90 days, and that within 28 days of that installation, I should contact them to confirm this. They'd also like to see a simple line drawing of the system at that point.
I know all DNO's are going to operate differently, but you may be lucky. Especially if you live in Somerset, and are covered by "National Grid Electricity Distribution (South West) plc"