We sanded off the cement overcoat with power tools. (took a few sheets of sandpaper !) and snapped on the indoor Polyfilla.
Must say it's lovely stuff to work with. Sands down beautifully, and so smooth. I put it in the sun to dry off before spraying and found the wood contracted with the heat of the sun. It was 28 degrees out there. Cracks have started appearing between the Butt joins and the wood strip cladding had rippled a bit. It could be due to the wood glue that I've used with the glue gun. I've now re- covered with Polyfilla again and will sand down tomorrow.
I'm now thinking that Epoxy is the way to go, either inside or outside is the question. I'll let you know how I get get on.