Just an update. The RCBO plan has been shelved having missed the offer so it is back to RCDs and MCBs. The garage CU will now be a RCD protected unit with MCBs the supply cable will come from an non RCD protected MCB in the main CU this arrangement should mean faults in the garage circuits should not affect the house circuits which was my main concern when considering RCBOs. The garage supply cable will either run down 50mm below the surface for 2m or run on the surface behind a kitchen unit. I got an MK CU with RCD and fully populated for £73, a couple of MCBs need swapping but I can do that when the main CU is replaced as the existing one is an MK.

Thanks for all the help and advice particularly Adam.

Richard