Sparks wrote:

Is there any value in having the house earth in the garage as well, and also
connecting this to the rod? - or is this a definite no?

A definite no. I believe you wrote you had the SWA armour connected to
the house earth at the house end, but isolated from the garage CU -
that's just what you want to make the garage a pukka TT system. If you
did connect the house earth to the garage rod, under fault conditions
(not necessarily in your own house, but for example another place close
by on the same phase of the same substation) you might end up with quite
substantial fault currents coursing through your earth wiring; not
really desirable...

I have just done some calculations for the house - shed run... using
http://www.kevinboone.com/cablecalc.cgi

House to Shed
Generated by CableCalc V1.0 (c)2000 Kevin Boone, all rights reserved
Basic cable properties:
Cable type: PVC-insulated two-core-and-earth 4 mm2 with 1.5 mm2 earth
conductor

Liar, liar, pants on fire ;-) You said in your original post it was SWA.
If 3-core (L,N,E all get an inner core of their own, armour earthed
Because It Should Be rather than to carry fault currents) then the earth
conductor's the same size - 4mmsq - as the L & N. If it's 2-core with
the sheath being used as the E conductor, you need to use the right
Table (or hope that CableCalc has access to its contents) to use the
impedance of the armour.

Voltage drop:
For a total cable length of: 25 metres

That's the second time you've said that ;-) I take it, then, that the CU
is more or less in the opposite corner of the house to the shed, so that
although the shed's only 4m from the nearest bit of house, the total SWA
run is 25m as you say...

Volt drop at end of cable: 8.8512 V when carrying 32 amps and cable
temperature is at maximum value of 70 degrees celcius

So at (unrealistic) full load, that's just about your entire voltage
drop budget eaten in this first 25m (conventionally we allow up to 4% as
cable loss - for the 240V supply that's 9.6V). Unlike the earth
impedance calc, this isn't affected by SWA-versus-T&E, since the live
conductors are the same size (4mmsq).

Then you take another 25m (and you lie again about the cable
construction ;-) for the segment from shed on to garage. But the calcs
from CableCalc believe this is a 25m segment from the origin of the
supply; so you need to (a) add the voltage drop for this segment to the
already-established 8.8V drop for the first run - putting you over the
voltage drop limit for drawing a heftier load in the garage; and (b)
establish clearly what the earth fault path actually is for an
L-to-E-conductor fault right at the end of the garage run. If that path
relies on the house CU breaker, you need to factor in the full 50m of
the combined run to do your fault-path calcs (which I now notice you
mention, at the end of your message, do assume the reduced-cross-section
protective conductor of T&E, while you know you have 4mmsq.

All looks OK to me, and that would be exporting the earth to the garage via
1.5mm cable (I have 4mm)

Though we're not actually 'exporting' the house earth to the garage, are
we? (The garage end is TT). But the house earth is what an L-to-E fault
at the end of that supply cable will go back through (and you've got the
worst-case 0.8ohm earth impedance of your TN-S supply to allow for in
calcultating the fault current, touch voltage, and disconnection time,
therefore...)

HTH, rather than confusing - Stefek