How best to fill a chase?

Hi, I'm just after a little advice on how best to fill a chase

I have a bag of Thistle one coat gypsum plaster, I have never used that before though I assume as it's called 'one coat' therefore I should be ok to finish, just I read it can be more difficult?
I have used finishing plaster previously and read that this maybe easier to get a nice finish is that true or should I be fine with the one coat?

I do have a box of Toupret filler, could I finish with that if the gypsum isn't suitable or would finishing plaster be the better option? I assume with the Toupret there would be a bit of sanding but it's a relatively small area so I could live with that, again I have only used this filler once on a small area....

The chase is no deeper than 2.5cm (a little less in places) and runs around 1.5M up the wall.

Thanks

I have used onecoat to fill up chases before. It's quite easy to use but finish isn't as smooth as multi unless you know how to sponge float. If I were you I'll deep fill the chase with Onecoat as a undercoat, and finish with toupret which is a fantastic product for surface finish.

Thanks Vicky that's what I'll do, just one more question, do I just need to wait the 90-120 Mins or so to dry before I use the Toupret or can it go straight on?

Toupret instruction says dry surface so I waited for OneCoat to dry.