Wall under Bay window

I am doing some renovation to the front room in my Victorian terrace.


The small 40 high area of wall underneath the bay window feels very loose and springy, on closer inspection it turned out to just be hardboard. When I pulled a piece away, I can see the unfinished brick behind it. (see photo)


There may have been some plastic sheeting back there too which has now perished. My original intention was to replace the hardboard with something sturdier, maybe plasterboard, or some extra thick plywood. Seeing the bare brick was a bit of a surprise, what’s the best solution going forwards? I don’t want to do anything which is going to cause a damp issue in the future.


Thanks in advance for any advice.

Looks like cheap thermal board, not cardboard , I would use insulated plasterboard.

Thanks I'll give that a go I think. Should be cheap enough.