Cheers for the response, Trixim... Does the 1/2mm up/down play thing make any difference? If not, I'll just seal the bastard...
The following information is all AFTER my personal recommendation: which is to replace the entire unit since it is damaged, as they are really not that expensive and it is a critical piece. Fuel is not something to take lightly, fuel and the fumes it vaporizes into are highly flammable. I do not recommend repairing fuel leaks. Just replace.
Now for the info:
From my personal knowledge it's possible for the plastics to wear down at those joints, it's not very high quality, but it still can be an O-ring.
By any change did you allow your tank to sit empty for a long time? This will allow the plastics and rubber seals to dry and crack as they are brittle when dry.
Finally to answer your question, just make sure you have a nice seal with the epoxy, once you smush some epoxy in there, you will probably stop the wiggle if its thats small. Again LET IT FULLY SET BEFORE USING IT AGAIN.
Take your time with this. Make sure you have a nice seal, without too much excess epoxy. If you need, i recommend watching some videos on how to use marine epoxy to make watertight seals! Check / practice until you are confident you can execute it perfectly.
I am so adamant about it being right because if you really mess this up, you should remove all the epoxy (just thing of what that would entail) before retrying.
That said, if you do this properly, it could last you freekin ages.