Ian Stirling wrote:

The easy approach is to put a 40A fuse at the close end, and parallel
all the wires.
Of course, this isn't very safe, as there are unfortunately joins in the
cable.

The presence of joints shouldn't affect your thinking here - after all,
all joints must be sound. However there may be a different problem: in
a situation like this you must make sure that each cables in thermally
protected in the event of an phase-neutral or phase-earth fault on any
one of them [see Reg. 473-02-05]. You'd need to check carefully that
the 1.5 mm^2 CPCs are adequately protected by the 40 A fuse.

My current plan is to common all the L/N/E, put a 13A fuse on each
live, and at the garage end, have a RCD CU, with a seperate earth spike.

Anything obvious that I've missed?

Hmm, well, the use of multiple devices for overload & fault protection
of parallel conductors is permitted [see 473-01-06, et. seq. and
473-02-05] but usually this would only be done in a high current
situation where there'd be monitoring to draw attention to the opening
of any one device. With what you propose, one fuse could fail, reducing
your affective cable rating without you being aware of it. So if you
must do it I'd suggest using a 3-pole (linked) 16 A MCB, so if one leg
trips, all cables are disconnected.

Quite frankly though, I think the best advice is not to be so
penny-pinching - go out and buy some 10 mm^2 cable and do the job in a
more conventional manner!

--
Andy