How do i prep this hodge-podge for plastering?

[FoofOfTheWalkingDead]

We're in the middle of getting a new kitchen and the plasterers come on Wednesday. I've been stripping lining paper for a week and just about finished. The problem is that there are multiple different surfaces to plaster over so I'm worried it will crack and come away.
There is the original glossy paint from the 30s, 40s and 50s. There is original plaster. There is drywall, some in not so good condition. There is new plaster over rewiring raggles. The worst part is the multiple sq ft of polyfilla that has been skimmed over much of the surface behind the worktops. Some of this was put over old lining paper. I am cursing DH's mate who was the 'contractor' on the reno he did just before I met him.
How do i prep this surface? Can I just clean it and roll on pva? Help!

We've got plasterers starting tomorrow and stripped off a million layers off paint and paper, I sanded all the walls with an electric sander to remove the last stuck on bits so they have a smooth ish surface to go at.

Is it a possibility to just put plasterboard over it all?

@Kjr33

Is it a possibility to just put plasterboard over it all?

Unfortunately, no. I wish.

@Summersdreaming

We've got plasterers starting tomorrow and stripped off a million layers off paint and paper, I sanded all the walls with an electric sander to remove the last stuck on bits so they have a smooth ish surface to go at.

I've been stripping paper for 3 days. I guess I'll be sanding for another 3😭. Did you seal it with anything?

We recently had some plastering done. The different surface aren't a problem as long as they are solid.

We had bits where a previous layer of plaster had blown so had to come off, but the solid bits from that layer could stay. We had bits where the original, more than a century old, lath and plaster was exposed, bit of brickwork, bits that the plumber and electrician had carvers out etc...

They do a first layer that is rough and levels everything up before skimming over it all. We had parts so thick that they took nearly a fortnight to dry.

Just started painting this weekend and it is looking great.

No just sanded. I say "just", we did days of stripping before the plaster fell off two walls, leading to those being boarded, more days of work, finally found a plasterer with space and no time to do anything but sand it! So finger's crossed. We move in on Wednesday .

It's hard to say OP would it be better taken back to the brick?

Thanks for the replies. We're mid-kitchen install so taking it to the brick isn't an option. I'm going to make sure all the paper is off and walls clean with any loose bits pulled off. The original plaster seems to be pretty solid so I hope it will be ok. Plasterers are coming tomorrow so there's just not time to do more at this point!

Update, our plastering went fine! It looks great, I'm sure yours will be too.

Great stuff, @Summersdreaming! We got ours finished last week and it looks good. In the end I didn't have time for any sanding. I stripped all the paper and wiped the surfaces with sugar soap. They just slapped the plaster on over all of it and it looks amazing. Can't wait to get it decorated!
Now I'm researching whether to seal the plaster or just mist coat. Must coat seems to be the winner according to Google.

BTW, do not put glue on the finished plaster, it will make the paint fall off

there are still a few plasterers who like to put PVA on the surface, but most of them have been killed by angry decorators.

Mist coat.
You can dilute 50/50 if you want.
I went for 60 emulsion 40 water first and then put another mist on at 80/20. Not sure the second one was needed but we were painting white on top anyway.

Thank you @PigletJohn and @tanstaafl! I am going for the mist coat.