Ran a little experiment to try what Phil mentioned, i raised the radial ring from ground-level to just below the balun, and used a shorter (3" or so) pigtail to connect to the radials as a test.
Pigtail between the choke centre-pin and and centre screw on the 5btv is exactly the same pigtail as before (around 3" as well..)
NanoVNA calibrated at load/feedpoint overlaid over NanoVNA calibrated at source/generator end (in blue) with 1:1 choke & 5BTV, RG-213:
So, the moral of the story is to keep the centre and braid connections short by either using a so-239 and a coax pigtail, or if using wires, keep them as short as possible.
Studying DX Engineerings PDF on the BTV this isn't mentioned explicitly, but one can notice all the intra choke/radial-plate connections are short, wether it's a direct so-239 to the btv itself, a bulkhead so239 on the radial-plate with coax pigtail, or a short tinned copper-braid from DX Engineerings own choke.
The proof is in the pudding :) Thanks to all who provided advice, this is a test setup, now i'll have re-design the pigtails more permanently, soon.