1995 600ss front sprocket
 

i have just chaanged the front sprocket on the above

when looking at it there is just the sprocket, a plate that sits in a groove with teeth counter-opposed to the ones on the sprocket and 2 allen head bolts holding them both onto the shaft

two questions

1. the sprocket moves forward and back on the shaft - like it should have a spacer behind it - normal?

2. what is the correct torque setting for the bolts and should i use thread lock?

There is somtimes a little paly in the sprocket. check the teeth on the plate tho and make sure they aren't worn.
The bolts are about 6Nm. Just nip them up. Threadlock is a good idea.

not just play neil - actually moves on the spline front to back a good 1/4"?

should theer be anything sat bbehind the sprocket engine side ie a spacer or something?

No spacer. The plate sits in the groove, and when new, most of the play is taken up by the "snug" fit. It's not uncommon for the groove to wear tho. I've seen more than 1/4".
Check the plate first. A new one would tighten it up a bit.