I'm changing the front sprocket on a Gk76A. There is a fair bit of sideways movement on the shaft when the circlip is in place. How much movement should there be on the sprocket?

They are pretty wobbly mate.

Just check the wear on the splines on the gearbox shaft, Suzuki 2 and 4 strokes do wear fast. RGV's wear really fast!
Replacing the front sprocket before it's worn out looks after the splines also. The sprocket should be 7.1mm ish thick if you have any verniers about, some cheapo sprockets may vary. See my recent post about chain/sprockets.
Make sure you change the circlips often also and have ths sharp edge of the circlip facing away from the sprocket. Circlips usually have a sharp and rounded side.