Currently I only have RCD protected circuits in my house (One for sockets &
fixed appliances, and one for the lighting (Tripping one does not affect the
other)

(I am planning on re arranging this, putting the lighting, freezers and
alarm system on non RCD circuits)

Currently the shed feed is from a 20A MCB connected to the "sockets" RCD
(30mA) via a 4mm SWA Cable.
This cable is about 25m, however the shed is only about 4m from the house.
The house earth has been exported to the shed.

From the shed CU, via another 20A MCB (Connected to the shed 30mA RCD)
another 4mm SWA cable then feeds a smaller CU in the garage with yet another
30mA RCD (Only three MCB's)
This cable is about 30m. (The garages are right up the top of the garden, as
far away from the house as can be!)

The garage is then earthed with an earth rod. (It is about 35m from the
house)
(The SWA cable's armour is connected to the earth in the shed, but isolated
in the garage)

My questions are...
Should the garage feed simply piggyback the feed TO the shed CU, eliminating
the RCD and MCB there?

Should the feed from the house to the shed be via a 100mA RCD, separate to
the rest of the house?

I assume the earth in the garage should be isolated from the earth in the
house/shed, as it's earthing arrangements are different? (House earth is
provided from the electricity company (TN-S) where as the garage has it's on
rod (TT)

Another question is, why do I need to TT the supply in the garage? Can't I
just use the house earth?
(My logic is, the earth provided by the electricity company is at least 500m
long (That's where the substation is!)

(Patio is currently being re laid, so if I need to do any rewiring, now is
the time)

Sparks...