Have a look in the Museum section of this site. On the 3rd page of the Mongoose thread is a Supergoose.
The family tree in the UK went Grand Prix, Californian, Supergoose, Pro Class.
I think the Expert frame was more or less identical to the Californian's.
As far as I can tell, a set of Supergoose stickers would make your Expert look pretty much like a Supergoose.
There are differences in the tubing material and probably the geometry and tube diameters, but as an ex-Californian owner I can't tell them apart without decals.
Have a search on this site for other threads about the differences between the framesets, and the components supplied with each.