Thanks for the replies.
I've found a bootblock writing tool I coded years and years ago so I've been using that.
The reason for skipping the first 32 bytes is...
Code:
dc.b 'dos',0
dc.l 0,$370
...that doesn't immediately start at the beginning when assembled to disk as there's an EXE header which needs to be removed.
I'm using DevPac to write the code and my little tool bwrite to write to disk calculating the checksum as it goes.
It assembles fine. If I comment out the first three longwords and add "move.l 4.w,a6" at the start and run my copperlist views perfectly fine but it doesn't work when applied as a bootblock.
All I'm using is a ReSourced X-Copy bootblock which has been modified very slightly.
Starting to pull hair out...