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Got a picture? Personally, I'd probably screw a square of plasterboard in the hole so its sitting just below flush, then fill over that with toupret TX 110 - one fill to get the gaps mostly filled, and a second to get it level and neat. You can bevel the edges to make filling easier. Putting a watered down mix of PVA on the plasterboard in the hole and on any exposed edges where you've cut the bevel will help prevent cracking too.
@Airhead will likely have a more refined method though ;)
Nef-
OK. My plan was expanding foam to fill the hole, cut back and fill/sand/fill etc
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That's pretty much my approach although it helps when you cut a piece of plasterboard out of a wall to keep it for the cover up, then you can use a batten behind the plasterboard and screw the piece you cut out to it. If you use a hole saw then its really easy. Sometimes you need plasterboard tape for the edges, there are some finer scrims available. I have started painting all these repairs with Gardz from Zinsser, just ensures that you will get a good paint finish.
I wouldn't touch the expanding foam with a barge pole. One of the least forgiving methods and it can expand in walls creating all kinds of problems.
Tenderloin
Airhead
internaly wall, plastered plasterboard and filling hole where light switch has been removed