[Closed] Sealing gap around boiler flue

I need to seal the gap around my boiler flue.
The weather cover is on the outside but if you lift it up there’s a gap into the house so I need to fill the gap on the inside and outside wall.

For the outside I was going to use some general purpose mortar (the kind you just mix in its own tub - about a fiver from screwfix). Do the job?

I was hoping to use that on the inside too to fill th gap in and then put some filler or decorators caulk (or something else?) as the top layer on the inside wall to sand it smooth once dry.

I’d Appreciate Some advice ..