Rob G coughed up some electrons that declared:

Thanks Bruce and Tim - I need to put a secondary TT earth at my garage/
workshop and was a bit gobsmacked about the cable size Andy Wade
mentioned. My house TT earth was installed by the local board aver 30
years and in all likelihoods needs inspection and probable upgrading -
note to self must do this !!

Rob

Aye - 16mm2 isn't that bad, being the standard size you'd need for any TN-x
main earthing. If you went for 25mm conduit where necessary (common and
available) and use gentle bends and not many of them, I don't think you'll
have much trouble pulling it through 2-3m worth.

Shorter lengths, 20mm ought to be OK, though I've only pulled 10mm2 thought
that, but no trouble.

Cheers

Tim