I assume that you have now done the calculation farmboy and realised you don't have to change the Earth conductor. You can use the old figures for a BS1361 fuse and you will see you are very safe unless the PSSC is very low, in which case you may need to fit an overall RCD to get a reasonable trip time in case of a short circuit (which is very unlikely on the tails if they are fitted properly). The main cutout fuse is for overall maximum demand protection, protecting the service cable rather more than in case of a short circuit, but it does that as well providing the PSSC is high enough. Some rural supplies have PSSCs in terms of less than 200A, so fusing times can be very long even with a direct short, then the main earth cable could get somewhat warm, but nowhere close to failure.

I assume that you have now done the calculation farmboy and realised you don't have to change the Earth conductor. You can use the old figures for a BS1361 fuse and you will see you are very safe unless the PSSC is very low, in which case you may need to fit an overall RCD to get a reasonable trip time in case of a short circuit (which is very unlikely on the tails if they are fitted properly). The main cutout fuse is for overall maximum demand protection, protecting the service cable rather more than in case of a short circuit, but it does that as well providing the PSSC is high enough. Some rural supplies have PSSCs in terms of less than 200A, so fusing times can be very long even with a direct short, then the main earth cable could get somewhat warm, but nowhere close to failure.