So I checked the fork and no schrader valve so its a coiled spring type I guess. It also has a +/- dial on the shock crown that states 'Preload'.

To set the sag I:

1. Made sure the fork was unlocked.
2. Tied a zip-tie to one of the forks and had it pressed up against the top of the damper.
3. Climbed on the bike and supported myself by leaning against a wall in my natural riding position.
4. Dismounted and measured the amount of travel for the zip-tie with me on the bike - 32mm.

I think that's about right. Its a 120mm (sticker on the fork states '120mm - Medium') fork and I read the sag should be approx 25% of the total travel of the fork (120mm). So that would be:

If 25% = 32mm then:

4 x 32mm = 128mm

Shouldn't that be close enough?

Thanks for all your help guys - the fog is clearing