"Martin Whybrow"

wrt solid/split HS bearings
when I went to fit the spindle, it was very tight, and when
fully home, was almost impossible to turn.

Keep hand turning, unless so tight that tht doesn't seem to make sense.

And
My question is, what would be the best way to adjust the bearings to size,
do they need to be line-bored after fitting to the headstock?

Hand scrape? A triangular (three-sided) scraper will work.

If you blue the bearings, and gently scrape off just the blue?
Frank Morrison