Thetford toilet help!

1966Splitty

Hi, i have just purchased our first caravan and before we give it a test run, I have been sorting out a couple of bits.

One issue I'm having a problem resolving though is the toilet. The sliding shutter gets stuck when trying to close it and then doesn't 'grip' on the closing slot when using the slider on the toilet. I can see that there is a second smaller arm (with the hole in it) that closes with the shutter but when it has been opened, falls off the back and prevents it closing. If I put my hand in (not too bad whilst it is still clean!) and lift it up, it closes again. Where should this arm sit and is there an easy fix? Hopefully the pictures will better explain!

Thanks in advance for your help and advice.