I've agreed to lay some vinyl flooring for my neice, over mainly
boards, which I'll hardboard over first. However there is a disused or
unwanted mat-well set in cement and IIRC, painted.

I was initially thinking of filling it with some kind of wood sheet
material but it occured to me that it could perhaps be better and more
accurately done by floating in some cement-based product. I can't
imagine this could ever be stable as it only 3 cms deep.

Any ideas on whether Unibonding and sand and cement or whether there
are any proprietary products that would fit the bill.

TIA Andy