Re-plastering painted window reveal...

Remove anything that's loose, and anything that might pull off when it gets wet (like wallpaper).

I'm not sure I have an accurate picture in my mind, but the area should probably have been lined with plasterboard rather than plywood originally. Personally I would probably strip out the ply, and depending what's behind, either apply a scratch coat to it (undercoat plaster or sand and cement depending on likelyhood of any penetrating damp), or fix plasterboard instead.

I may be able to get away with just removing the skim bead on the front top of the reveal and screwing 9mm plasterboard to it (after removing loose stuff) then reinstating some new skim bead.

The sides should strip back to the S&S render, I guess which can then be skimmed directly.

The property is very old and its not clear to me if plasterboard would have been in common use when the ply was put up. I don't want to pull it down though as the rubble infill in the wall might cause a bit of a mess.

Cheers,

Alex.

Scrape the rendered bits to get rid of most of the paint, tack some EML to the ply and give it a thin bonding coat.

Thanks for the suggestion Rob.

Being a plastering newbie ... Err what's EML?

Cheers,

Alex.

Expanded metal lath - a sort of wire mesh, the same as the perforated bits that hold on metal angle beading, but in a flat sheet.

Aha, seen it. Did not know what it was called.

Cheers,

Alex.