I spoke to the BCO today and the outcome of the conversation is that he insists I include a Surge Protection Device (SPD) in the CU and looking into the calculation required to avoid inclusion and the faff collecting all the info it is probably easier and gets the job moving just to fork out the £44 and be done.

So this is hopefully the final layout of all the protection devices in the CU:

https://www.dropbox.com/s/vdv6ch8q1p...yout.docx?dl=0

AS the SPD is not connected to the busbar I have included it at the end of the first section containing the Garage MCB and the two RCBO devices. The issue is that it needs feeding from a MCB 16 - 32A. The manufacturers recommend a 32A ring main. So does it matter which of the two devices as indicated on the diagram is used?

Finally, I have added a separate isolator switch and planned initially to feed the tails from it into the bottom of the CU but would now like to fit a socket below the CU and with not much room to play with it would be convenient to feed them through the top? There are suitable knockouts in the top and the instructions say top feeding is possible but with the convention being to bottom feed and CU is situated below a kitchen worktop would anyone raise any objections?

I know I should have asked the BCO but had other things on my mind and I do not want to get into another conversation that will cost me money!

Richard