I am guessing the 40 amp RCD is the main switch? Do you know what is actually feeding the garage supply from the house?
Absolutely nothing wrong with running radials for socket circuits (as long as the final installation is tested and passed) but since you are going to be having a lot more stuff onto the circuit you might as well upgrade the MCB for sockets to 20 amps as they should be anyway.
You can certainly spur off one of the sockets (fcu) to feed the garage door but space permitting on the board, why not just run dedicated circuit from the fuse box off a 6 amp MCB especially just for the garage?
That's the way I'll do it anyway, and that helps with 'Diversity'.
Hope the above helps.