I'd heard a rumour that you could notify MDL that you would be away from your mooring and get a credit if they let someone else use it. Unfortunately no-one at MDL seems to be aware of how it works or even how to do it!
Well I think I found it on MyMDL!
1. Simply login to MyMDL with your email address and your account number.
2. Choose Booking from the main menu
3. Choose Book out from the dropdown
4. Enter the dates you'll be away
5. Hope they use your berth and the visitor leaves before you get back :-)