Having extreme issues trying to get my HG1 9.0 to rotate.

On an older Severne 490 mast. The bottom 2 battens can be encouraged to rotate but the top 2 are really tough. Luff pocket is a bit saggy.

Appears no spacers or metal things on most cams (2nd hand sail).

When rigging, the cams are very fond of popping off until full downhaul achieved - should I be adding spacers ?

Cheers

Sounds to me like most likely you're problems are due to the hyperglides being designed for severnes apex masts which over alot of their length have a smaller diameter than normal sdm masts including old severne models
So this would mean that your cams in particular the top ones will have more pressure than designed
Bit hard to fix, could try filing down the cams to shorten them but that probably wouldn't solve the whole problem

I know this can be hard to do for most of us, but see if you can try different masts. What I think you are describing is a bend curve issue. If the mast is too stiff up on the upper cams, they won't rotate easily.

Thanks for the suggestions I will try another mast

If you haven't done so already contact Severne. I think most sail makers would be able to offer some advice.