Thanks Nejad. The gasket, which does look to be PVC looks intact. The problem seems to be that when i push the plexi on hard it pushes the gasket out so it can be seen in front of the dial. Will try and re-seat it another way to see it this helps.
Honestly don't know how this has happenned. QC seemed fine when i got it (from Homer whilst he was around)I may have given it to my 2 year old daugter to play with and only knows what she did with it!!!
I was thinking of replacing the domed plexi with a flat version. Anyone got any ideas what size glass I need to source or where to get one. I am in the UK by the way.