If the walls are relatively plumb I'd strip what's left on there and screw some wediboard or similar to the wall then tile straight onto it.
One wall is definitely bang on. The other I don't think is. Also, it's incredibly dense concrete underneath (I think), will I have trouble screwing on the boards? I did think about going down the aquapanel route
You can install it with adhesive.
@TooTallTim started
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If the walls are relatively plumb I'd strip what's left on there and screw some wediboard or similar to the wall then tile straight onto it.