Thanks for those replies, which have provided some useful pointers.

I can get to the back of the hole so I think I may get some fibreglass matting to provide a solid base and then go for Milliput or car body filler.

I suspect Milliput may be better as it gives lots of time to work it before it dries.

My other half lives in the flat upstairs so it's only me that has to live with it!!