What sort of load is going to be in the summerhouse? While I'm all for over-engineering to "be on the safe side" and to allow for future growth, I'm wondering what's going to be in there to warrant a 40A subfeeder. A 30A subfeed with 6mm SWA would still be within voltage-drop tolerances, even at full load.

If the hosue is PME, then a TT system with separate earth rod at the summerhouse might be the way to go (yes, I'm trying to put aside my personal dislike of TT arrangements [Linked Image] ).