Removed skirting board, to this..

ThisIsWeird

To first fill the voids with bonding, and the visible parts with a further filler/skim is certainly the best or proper way, and shouldn't be difficult since you have the flat wall surface to guide you for levelling (tho' do check the wall is plumb at that point - I've come across some which go 'in' at above skirting level, and also that there isn't a raised edge there). But, for anyone who wishes to avoid this messy job, there are alternatives.
One is to obtain a timber batten of slightly less thickness than the void, and grab-adhere it horizontally along there, say an inch up from the floor level. Good dollops of grabbie, and press the batten into place using a straight rule placed against the wall. Then glue & pin the skirting into place.
Or, expanding foam as mentioned before - tho' it would need to be able to expand enough for the job. For this method, I'd first cut some wee spacer blocks of slightly less thickness, and grab-adhere them into place every ~600mm or so, again pressing them into 'level' using a straight rule as before. Allow to set, and then your skirting should be easy to position.
For any adhering or bonding, I'd recommend first SBRing the dusty brickwork. 
That leaves only the visible gaps to fill. A multipurpose filler will do this easily.
If the visible gaps are longer and or numerous, that is what would tip me over into bonding-plastering it all. For a full bonding fill, what do folk suggest as a spacer against the floor - isn't a wee gap recommended?

stuart45

You don't really need a spacer. Just don't push it tight to the floor, and cut off any that's touching with the trowel once it's into it's final set. 

rob7475

There's been a picture rail around the room at some point - can you tell how well that's been filled? In all honesty, if you want a good finish, you're going to have to get the room skimmed.

When I renovated our 1920's house, I thought I could get away with filling the bad bits but even when wallpapered, it didn't look perfect. When rooms have been due to be redecorated, I've ended up skimming them and they look much better.

Learning to plaster is a very good skill if you can master it.

FreeBear

ThisIsWeird said: One is to obtain a timber batten of slightly less thickness than the void, and grab-adhere it horizontally along there, say an inch up from the floor level. Good dollops of grabbie, and press the batten into place using a straight rule placed against the wall. Then glue & pin the skirting into place.

Fixing a batten to the wall at half the height of the skirting is not a bad idea especially if you have hollow terracotta blocks. Gives you something solid to screw in to when fixing the skirting - I'd still use bonding plaster to fill the rest of the void. Expanding foam smacks of bodgery.

ThisIsWeird

FreeBear said:

Fixing a batten to the wall at half the height of the skirting is not a bad idea especially if you have hollow terracotta blocks. Gives you something solid to screw in to when fixing the skirting - I'd still use bonding plaster to fill the rest of the void. Expanding foam smacks of bodgery.

Tbh, I have no idea how far it expands, and whether it would be strong and tough enough when expanded to fill the ~15mm gap that this appears to be. So I'd def fit spacer battens/blocks first in any case.

The joiner who fitted the skirting boards to our extension a few years back, did use exp foam adhesive for this, and the 'boards are totally secure and with no gaps, so I presume it's an actual ok product to use in general. 

I might be tempted to use it myself, but would fit the correct spacer battens first just to make the 'plumb'ing aspect of fitting the boards assured.

stuart45

A lot of chippies use a low expanding foam to fix the skirtings.

danrv

Hoffnung said:

stuart45 said:

Hoffnung said:

Ah, I thought we could have just patched this up at the bottom with some sand and cement or bonding, as we intend to just line it/wallpaper over it. 

Maybe back to the drawing board 😳

It depends on the condition of the plaster. If it's sounding hollow in a lot of places it might need hacking off. Skimming only gives you a smooth surface for decoration, it won't solve blown plaster issues. 

I thought this would be able to be quick tidy up at the bottom ready for new skirting board and then line the walls and wallpaper them. 

Certainly doable. Maybe not quick.
I'm doing similar work in my property using the 'make good' approach and learning to plaster at the same time.
There's some very good products about and shouldn't cost a lot. Toupret FlexFiller is useful for any movement cracks.

Sand/cement - plaster if needed to build up the gaps at the bottom. I'm using bullnose MDF skirting of suitable height all round.
Cheap grab adhesive I find is all that's needed and a lot of heavy weights to push it against the wall.
I use a circular saw for the mitres.
 

flashg67

Personally I'd use bonding then skim on the missing bits. I've tried the one coat plaster but found it takes ages to dry when used to fill deep sections. You could even use wood battens at the bottom to fix the skirting to then plaster what's left above