Over the last couple of outings I have noticed that there is a leakage problem with our toilet cassette. When I remove it, the recess in the housing is filling with waste water. This appears to be coming from the slide assembly on top of the cassette. Does this sound like a new lip seal is required? Are they difficult to fit? I have removed the slider and can see a seal, but no screws to release, so has the seal to be "dug" out then the new seal pressed into place? By the way, this is one on the cassettes with wheels and a large circular section on top. Any help or advice would be gratefully accepted.