Here is a quick example
supply goes into top of RCD, L for Live, N for Neutral, your earth core into the earth bar
then a example of how a circuit would go into the MCB, just follow suit after that (ofcourse you will have two cables going into the 32A breaker/bars for a ring final circuit, just put one set of cores for an example of where they go)
Busbar is all you need to complete this now, it goes between the bottom "L" of the RCD, and all the MCB's at the bottom also. (See below spoiler)
Where does the 2.5 come from? This needs to be your first concern IMO. It needs to be traced and find out where it comes from/what it's connected too.
If you take the front cover off the C/U and see if there's three brown cores connected into the top of the MCB that powers it, or just two. If it's two then it would indicate that there's a switch somewhere (in the kitchen?) for the garage supply, or there's a junction box behind the wall (which I wouldn't be inclined to be the case, as it's a new build). If you take the cover off and there's three brown cores in the same MCB, I would think that one of them three cores goes straight to that outside 4x4 junction box that the SWA is connected into.
Yeah that looks all in order, as you suggest may be worth a shot looking for the third cable in the kitchen sockets, but it's entirely upto yourself, I can spend hours finding cables and then at the end of it none the wiser, it could be in a junction box behind the wall for all we know
It would definitely suggest a pup was second fixing - I'd say look in the CU, you may be pleasantly surprised.
We have the 2nd year apprentices 2nd fix whilst the spark does the board. He then has to open everything up and check the work, if he's not happy he pulls it all apart and reterminates everything. The lad then spends the next day stripping cable in the stores
It's not a quick method and has pushed some rewires out by a few days but they tend not to do it again - stripping 1mm with a pair of tail shears for 8 hours is no fun lol
