It would be fine, so long as you do it properly

You can't just pop a load of filler in there because it won't go off properly and will end up sagging and not taking hold. You need something to fill up the void as best as you can and then layer the filler until its flush with the wall. Then sand it and it will be flat. You just need to ensure that when you use the polystyrene you trim it so that it gives you enough clearance to ad those layers of filler in. Add a filler layer, let it go off and then add another until you filler the hole. Go to a DIY shop and buy a filler blade (usually a plastic rectangular shaped blade 30cms x 10cms ish.) will cost you a tenner probably. You'll be able to get a fairly smooth finish, especially if you wet the edge of the blade slightly.

Posted By: I Am Hoot, Jun 20, 21:27:38