BigWallop wrote:

Springs Toggle fixings or plasterboard fixings:

http://www.rawl.co.uk/Products/lightmain/cavity.htm

should do the trick.

[a] They're completely useless on lath and plaster, which is
what the OP has.

[b] Even if it was PB, I still wouldn't want to hang a big
vibrating fan on them, because they will eventually fail.

--
Grunff