Blocking off old light switch

Hi - I am moving a light switch and wanted to find out how other people have blocked the old one off.

I am considering just filling the bulk of it with cement, then some plaster board nearly flush with the rest of the wall, then skim some plaster over the top.

I will be plastering elsewhere anyway, so it is not much extra effort.

I am concerned that I will get constant cracks around the area where the old switch was.

Also, as it is such a small area, will the plaster hold?

Thanks in advance for any replies.

Is this a blockwork wall? If so the best way to make good would be to remove the old steel back box and fill the void with sand/ cement mortar, leaving the surface slightly recessed. When dry finish with skimcoat. To counteract shrinkage it might make sense to use a bit of fibreglass repair tape over the join (raking back the ex. skimcoat and feathering it as you do). Bonding can be used instead of sand/ cement as it dries quickly and is easy to use.

Thanks for your reply Carpenter!

Yes, it is blockwork - I have already removed the casing.
You dont think plasterboard is required?
I thought it would be better than plastering over the cement (dont know why I think that, I assume it is called plaster board for a reason).
I am only a part time DIYer - dont have any bonding lying about, but I do have half a bag of cement, so will try to use that up.

There is no benefit to using plasterboard; sand and cement is the best job. Try and allow the mortar to go off for a few days before skimming, to avoid shrinkage cracking (although it probably inevitable!). The technical term for what you want to do is "dubbing out"- filling holes and gaps in masonry construction with sand and cement before plastering.