Hi Julia,
Just done the same job on mine.
First make sure nothing else is stuck in the hole, I prod mine out with a chopstick including turning the pump which I can see at end of hole.
Also, the bit that screws in has two lugs that stick out just beneath where you hold it. check they are in the two small cutouts where you screw the filter in.
Use a torch and look really carefully what is stopping it screwing in. The lugs may be the answer. Hope so.
Take care
Ken
Ken Wyatt
June 2008