Well - I've made a longer fret-filing tool that is a bit over the fretboard length.
My existing file I'm also still using.

So - made a notched straight edge out of a steep ruler.
got the neck flat - and noticed that the fretboard was possibly not dead flat...

So I put the neck in a very slight back bow (as per the video)


Then levelled the whole fret board with the longer levelling tool (used 240G sandpaper) frets one and two were barely "touched".
I then put tape on 9th fret and used the shorter tool (same grit) over the frets 10 to 22, making sure that none or very little was taken from frets 10, 11 or 12.

tidied each fret up with 400 grit sandpaper
Took off the shoulders of each fret with 600 grit.

Restrung, tuned up and adjusted the neck to a slightly forward bow and tuned up again....
capo'd at the first fret, the Low E string is about 2 mm higher than the 12th fret.
Capo'd at the first fret and fretted on the 17th - the Low E string is about .5 mm above the 7th or 8th fret
Capo's at teh frst fret - the string is about XX mm higher than the 22nd fret.

Normal "playing" - no buzz

If I use my thumb on the low e with a "bit of force" - buzz.
Buzz is a tiny bit "easier" to get when fretted on the first fret. Fretted at the second and no buzz - so I wonder if the second fret needs to be fractionally lower than the first?
fretting all the way down the neck and a very slight buzz happens approaching the 17th fret.

There is some improvement (I think the initial one was a bad levelling effort!) but there is a very slight buzz on occasion.

So if I repeat the process (this time taking a tiny bit off the second fret and a bit off frets 16 to 22) I'm hoping fr no buzz.....