Thetford Cassette Holding Tank Flap/Disc

You know it's not going to be a good job when the cassette flap/disc shears off and falls inside ! ! ! The lung on the back of the sealing disc broke off, looking at it it's not very surprising as it's really not very sturdy. The £7 replacment is a redesigned part and looks stronger also it only took 5 minutes to fit. Has anyone else had this issue ? ? ?