Plastering advice please.

Yup nothing wrong with that if all the existing surfaces are at least a plasterboard thickness depth less than the current plaster finish. You will need to add a couple of extra uprights in the openings to make it stiff enough.

If you can live with the "hole" effect, then you will save money and time. You'll also need to check what's under the floor, as a block wall won't go very well on top of a suspended floor.

It will look like a bodge, IMHO, though unless you bring the thickness up to match the ret of the wall.

It will also be less soundproof than a proper block wall.

Totally - plasterboarding would be a perfectly correct solution. You have the makings of s stud wall anyway with 4x2" timbers so go ahead...

You might want to stuff it with glasswool to give an acoustic block but that's upto you.

The uprights are 20mm inset from the surrounding plaster surface so plenty of room for plasterboard. With modern plaster finishes I don't know what the usual bonding and skim thicknesses are when using plasterboard.

The idea is for the finished surface to match the surrounding walls (using a combination of plasterboard and plaster) so there should be no hole effect. The partition sits on the concrete raft that supports the front half of the building so no stability problems.

While sound transmission is not really an issue as it is an internal wall in a quiet building I was intending to put some celotex in what were the glazed areas and the boarded area below them 'just in case'.

You wouldn't normally use bonding, even skim coat is optional. You could double-skin it with two 9.5mm layers to come close to you 20mm

To blend the area into the surrounding wall I imagine that a skim coat would be necessary, possibly extended over much of the surrounding walls so that no trace of the old partition remains. Given this is a skim coat of 1 or 2mm going to be enough to achieve a really good and stable surface?

Depending on what's above the ceiling at that point, the frame may well be load-bearing - so it's probably best to leave it in place. Covering it with plasterboard and skimming it is a perfectly reasonable thing to do provided you can make the finished surfaces line up with the existing wall.

As I'm sure you're aware, with the glass replaced by an opaque wall, it will be a hell of a lot darker in the room. Is that what you want?

The wall includes the partition is definitely not load bearing.

I did consider the potential reduction in light from the partition and I fairly sure it is going to be a problem. The glazed door is being replaced by another glazed door so no reduction there. The glazed panel on the left of the photo let relatively little light into the area as it faced onto a wall the other side of the corridor. There will be some loss from the panel on the right side of the photo as that faced down the hallway to floor to ceiling windows and a glazed door, but there is a window on the opposite wall less than 4 metres away and I am happy that the remaining natural light will be sufficient (I blanked off the partion glazed panels with a thick dust sheet to test it).

Good idea - that is undoubtedly cheaper than celotex.

The existing walls are the reference level. Build up the plasterboard so it's a couple of mm below the existing walls, skim the PB and feather out no need to skim the walls as well.

On PB yes, no problem. The double layers of board will also be good for reducing sound transmission, stagger any joins in the boards and you can get PB that is better at reducing sound transmission as well. I'd still but glass/rockwool insulation in the voids though.

You don't usually using bonding at all - just a thin skim of multi or board finish. In your case you may find a couple of layers of 9mm board and then skim works well.

But probably more soundproof than the existing glass!

I am just wondering if it is already studwork above the door and windows.

Wots with all this skimming? is the existing wall plasterboard? Just pack out the framing till one sheet of plasterboard is level with the existing and join the seam with the normal instructions for plasterboard IE tape and finishing plaster. and if the existing wall is plasterboard the textures should match.

How's celotex relevant? Not looking to insulate it and celotex is not an optimal acoustic insulator

The existing wall is rendered blockwork.

From a quick test drilling it is blockwork.

I was thinking of using 2440x1200 sheets so no joints to worry about. I will use rockwool insulation.