need a new diaphragm in flush system?

have unscrewed the fulsh system and discovered howit works. Could buy a new diaphragm but have found that where I unscrewed the flush section from the outpipe in the cistern, it is imossible to screw it back in. Have been told that I could replace the system but it a low level toilet which has the pipes encased in the ceramic piece under the cistern. Any suggestions for a girl willing to have a go and concerned about price of a plumber if I can manage it myself.