robgraham wrote:

[...]

The details are that I can work out is that it is a 2.5 T & E that runs
I reckon some 25 m from the CU to a shed en route to the garage which
is some 8 m further on - at least the outer wall of it is. The CU fuse
is 15A. There's a fused splitter in the shed (brick !), one feed
supplying the lights and sockets in the shed and attached lean to
greenhouse, and the other the garage, both I think are 15A.

Is the greenhouse timber- or metal-framed?

The old man's memory isn't as good as it used to be - or more likely he
is suspicious that I am going to condemn his work(!) as he is not being
that forthcoming on how the cable runs underground from the house, but
as far as I can see it is in a plastic pipe about 25mm dia. with an 8m
u/g run to the brick shed from the house.

The loading on this possible guddle is light - some lights, a deep
freeze, beer fridge and a wood turning lathe are the only things in
regular use; the workshop is elsewhere and seperately fed (6mm, etc)
and that's where the heavy loading is.

Condemning this garage supply would be met with severe objection, so
how can I make it safer ?

Is the house earthing TN-S or TN-C-S (PME)? What's the floor like,
especially near the fridge, freezer and lathe (Are these items in the
brick shed, or the garage? Are the floors damp? Are there any metal
service or waste pipes entering thro' the floors? Is there any RCD
protection anywhere?

What aspect(s) are you actually worried about? The length limit for 2.5
mm^2 T&E on a 15 A fuse is 35 m, so there may be a concern there,
especially if it's pre-1984 cable with a 1.0 mm^2 CPC.

--
Andy