Thanks Sunray and yes, I was thinking of a 2P RCD + 63A fuse. I haven't yet found a 2P 63A RCBO.
Below is a wiring diagram of what I envisage based on the discussions so far. Couple of NBs:
(1) The earth connection from the genny to MET will be via the 32A connecting lead so not permanent.
(2) I haven't yet found a 2P 63A RCBO but no problem with a 63A RCD. So may revert to the RCD + 63A fuse alternative.
I have two further questions:
A) I presume the RCD/RCBO should be type AC?
B) It seems logical to place the RCD and fuse with the RCD closest to the genny.
Any further comments?
Ref. A) above, this is a 63A RCD that (from description) can also be used as an isolator but doesn't indicate what type it is - A/AC/F etc.
https://www.tlc-direct.co.uk/Products/WYWRS63slash2.html
View attachment 262885
I don't recall if there's an existing RCD feeding the garage SWA, if not (assuming there is a RCD in the garage CU) use a 63A 2 pole MCB.
63A RCBO's are available, I did a google and they are easily found, some are expensive!!