How to fill this hole in the wall?

[Lentilpot]

Mainly - what type or filler and do I need to seal the block with PVA first?

The whole thing is about 10-15cm across.

[MikeUniformMike]

I'm not a builder but I would tackle it. I'd use interior filler after a coat of PVA.

[FleurWeasley]

Thank you for your advice! I’m definitely going to tackle it! It will be obscured by the new (properly installed, thanks previous owners!) curtain rail so doesn’t have to be perfect.

Can I use craft glue PVA or does it need to be the proper one sold near the polyfilla?

[MikeUniformMike]

I would use the proper stuff just in case. It's probably cheaper than the craft stuff. Polyfilla should be ok. Wilko and Screwfix are good for such things.