Stupid plaster on stupid walls

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Hi I'm currently in a mad decorating mood and spent the past few days planning on stripping my living room walls. I did start yesterday and found 3 deep holes that when your hand is near you can feel the cold air and wind from outside.
It's a council house and I have booked in for someone to come out and check as with how deep it is I'm not comfortable with trying to fill them myself.
As I've been stripping more tonight there is some newer plaster underneath but some large chunks have come away leaving a ridge that takes up half of a wall length. I'm not sure how to get rid of the said ridge and I'm worried that when I put lining paper up and paint it will be really noticeable.
I was wondering if anyone had any suggestions on how to sort this please as I have no idea.
I have included pics as I'm not sure my description is that great.

I think you're just going to have to get it re-plastered. Sorry

Yeah if the plaster has come off I'd expect the only thing you can do is get it replastered. Looks like the layer that's come off is too thick to cover with lining paper.

Ah balls :( thank you for the replies will show whoever comes out and see if they will do it or of I have to sort it out as it was done before I moved into the property.

It certainly looks like the hard skim has come off.

A common cause is the use of a hot steamer. Holding it in one place overheats the plaster and makes it burst off the wall.

If the backing coat beneath is firmly attached, and does not sound hollow when knocked with the knuckles, a plasterer can easily skim it smooth.

I used a sponge and water as that was what my grandad used to do when I was little. There's a slight gap between the bottom bit of the ridge and wall but not sure if that happened when I pulled the paper off as there where pieces bigger then my hand stuck to the paper.
I have tapped wall about 6 inches up from the bottom of the ridge and it doesn't sound hollow.

If finish plaster is skimmed on top of old plaster, it will break off.
It was probably the wallpaper holding it on.
Unfortunately, there is no easy way around this.
You could skim over it and hope for the best when you paper over it.
Long term, it all needs to come off. Sorry.
I'm guessing that your house isn't a new build? 1960's?

It's not a new build no I'm not entirely sure how old it is.
I have someone from my housing coming to look at the holes on Wednesday so will show them the ridges. Thank you for your reply:)