All sorted thanks, I made! Some dial gauges that attatch to the cams to give a fixed visual marker,

Jubilee clamps an 5" nails.

Content protected by owner... can't see anything!

I'm useless at this, should be ok now?

How do they work then. Where is the reference point they are pointing too. If you are relying on them being visually vertical before and after that is not accurate enough.

I got them both vertical at the point when the crank was against the stop, checked with spirit level. It's surprising how small a movement you notice.
I had to move the crank back a touch to fit the belt, then when moved the crank forward the tension the slack side of the belt was right checking the cams hadn't moved. I spun it over by hand without the plugs in before starting, it's running as it was before.

Also, thinking about it again afterwards, the minimum you can be out would be 1 tooth on the belt that is around (roughly) 9 degrees, that is quite easy to spot. With no way of locking the cams I couldn't think of another DIY option.

Simply do what I suggested in an earlier post i.e. "Simply mark all the pulleys with a suitable product before you remove the old belt. After fitting new belt make sure all the marks line up exactly as before.", its a commonly used method.

Caterham Zetec
Did that as well, belt & braces.