Hi Neil, welcome to the club. If your clutch is original it works on a quick thread which can wear and reduce the lift. Have you got a copy of “The book of the Scott”? It’s useful. I’m sure someone more knowledgeable than me will chip in sooner or later. If it is the quick thread there are mods which work better.
I think you should have a blind head motor with a water dome not a detachable head (I’m no 3-speed expert). The dome is secured by 2 rings around the plug holes which screw down. They are prone to leakage. If it doesn’t settle down with use you may need to replace the gaskets and check that the dome isn’t warped.
best of luck, persevere!