These bungs can be quite problematical; Roycruse couldn't get his out How am i gonna get this undone ???
but at least when he did it was dry inside. I have tried to undo mine but haven't been able to yet, but I had better check considering how much water you had in yours.
Ribcraft fit a transom bung as standard to the 4.8 and I ordered one for my 5.3 but they forgot to fit one. However I've not missed it though as I always pull a tarpaulin over him to keep the rain off.
Keep us informed, yes, Ribcraft are difficult to contact... (but it is Christmas and the London Boat Show is on in a few days) but when they do get back to you I'm sure they will be very helpful, as they have been with me.