Filling in hole in internal wall

[livinginhope]

The attached picture is of a hole in an internal wall - it's actually the wall of a built-in cupboard at the top of the stairs. I'm curious what the construction materials are likely to be as I don't recognise them. It's a ~1930s London suburban house. What would be the best way to fill in the hole? And is it likely to cause any problems if I remove the nails?

(Doesn't need to be a perfect finish because it is only a cupboard. The hole has been there as long as I've had the house - so at least 15-ish years).

(For scale: The hole is about 10cm across).

 

[Wayners]

2kg box of toupret.
Do it in 2+ goes giving it an hour or so for first mix to set a bit so it don't slump out.
Try not to mix more than you need or you will be throwing away good filler

 

[foxhole]

Looks like plaster on clinker blockwork , standard in many 30’s properties .
Nothing special required just a good filler .