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The thing that does stand out is the inconsistent use of grey and black for the CPC

Does it matter which wire is used for earth as long as it is consistent? Obviously, brown and black are the normal colours for live and neutral, so it makes more sense for grey to be used as earth.
From memory, the SWA with a grey earth wire is only used on circuit C1 to connect junction boxes J1, J2 and J3. There's a 3 way resin joint kit near J1, which takes the SWA from the garden wiring box and splits it to J1 and J2. J3 then connects to J2.