Actually, I've just had a thought. I may have a short length of the
original SWA knocking about that might reach the ground. I could start
the extension with that and bury the join to the 2005 SWA. What do you
think?

Given that the likely outcome of being discovered is that you get a slap on
the wrist and have to pay for a normalisation notice (probably at the same
cost as the original certification), I'd say do it properly in modern (even
harmonised) cable and then leave it to your conscience as to getting it
certified. However, do make sure that you've properly tested the cable,
including an earth loop impedence test. If you don't have the equipment to
do this, submitting a building notice might be a reasonable method of
getting the test done anyway.

Christian.