Probably a stupid question but should valve clearances be measured between cam lobe bottom and rocker arm top or am I approaching this completely wrong as there is naff all clearance between lobe and arm after about 5mm in.

You need to put your feeler gauge between the shim and the rocker arm. Are you using a haynes manual?

It's an NC24, there is no haynes manual and no shims.

The feeler gauge should go all the way in with light drag if you have the right clearance.

You can adjust one of the tappets really loose to check that your feeler gauges are level and your not hitting anything else.

Get a good luck at the area, been a while since ive adjusted 24 valves but i seem to remember the gauge is easy to hit other things

its a nightmare, everytime I try I get about 5mm in and hit something I might try bending the feeler gauge blades into a right angel to hopefully make things easier.

Undo the locknut and back off the adjuster. Slide the gauge in, moving it back and forth. Slowly do the adjuster up until you feel a slight drag.

I find it much better to back off all the clearances if they are the locknut type and start from scratch