Sparks wrote:

Okay, if I was to connect the garage to a 20A MCB, connected to a 30mA RCD
in the shed (No not Ideal I know!), would I still need to calculate from the
house?

With an RCD at the shed end, we can ignore earth loop impedance stuff,
as the RCD will cut out given an L-E fault. You're left with the voltage
drop needing calculating for the full 50m run (and the 4mm T&E figures
are reusable for 4mm SWA, neglecting differences in temperature rise);
if you do shorten the CU-to-shed run, and since you don't 'really' run
heavy stuff in the garage, you're likely to be OK (but a calc wouldn't
hurt).

If not, what would you suggest I do (I Really don't want to have to replace
the cables!!)

Of course you don't (who would?)...