See photo for example. Especially that bit of wall on the left.

Our rennovation has ended up with some external walls whose inner face has been a bit damaged.
In some places the plaster and render (plaster skim over what looks like cement render) have come away cleanly but in others, especially where the wall had been patched with aircrete block, removing the cement render has ripped up the surface.

I am wondering what is the best way to re-finish these walls. I don't want to board over so I presume I am looking at some sort of render to fill the space and then plaster over.

I'm guessing we need something a bit more high tech than cement render like we removed?

Damp inspection reveals a tiny bit of damp in a couple of walls but just in the bottom few inches. It will be treated but I still don't want to use something non-breathable and have the UFH just drive damp up the wall. And given the state of the wall I am wondering if a strong render is required to help hold it all together and minimise new surface cracks. Or should I use a lightweight rennovation plaster all over (it would end up pretty thick in areas so may take quite a lot).

Any recommendations?

For lowest cost, sand and cement follower by a finish plaster, otherwise Thistle HardWall + finish.

Since you're plastering over aerated concrete blocks, the wall suction will be very high, so you'll either need to thoroughly dampen the wall first or use GypPrime primer. You could have used Browning, but Thistle were due to stop production around the end of last year - guess they have.

You'll also want to carry the skim over the remaining good plaster to avoid visible joins. Provided the paint is well stuck to the plaster, you can clean it with detergent, wire brush it, and prime it with Thistle Bond-it. Otherwise you'll need to strip / hack off the paint.