Managed to get the 4 allen bolts out, and the external part of the gauge... but the back bit parted company and disappeared back into the tank....

Tried to fish it out but it wasn't having any of it..
Never mind, thinks I , just put it back together and forget I ever started the job...
Now it leaks like a good -un from the back of the gauge. The original plastic washer is back in place , so I'm assuming some form of jointing compound would be of use here..
Any suggestions before I pull it to pieces again!