to the forum Andy. Pity you had to join up because you've got a problem though!
A little increase in free play is normal as the engine/gearbox/clutch warms up. That's one of the reasons why an adjuster is provided on the clutch perch. If the free play gets excessive it’s an early sign that the clutch is overheating.
What happens is when the clutch gets hot, the plates expand and the clutch pack gets thicker. As the pushrod assembly passes through the clutch pack, this effectively increases the distance (gap) between the pressure plate and push rod, hence the lever goes slack. If you continue riding like this, the clutch starts to drag more due to not releasing fully, gets hotter and the situation gets worse; eventually it won’t disengage at all. When the clutch pack cools down the free play will usually return to normal.
If you do a lot of off road low speed riding where clutch feathering/slipping is required (conditions where the clutch pack gets hot) and you are adjusting the clutch free play when the bike is cold, then I would try and set it to have as little free play as you can get away with. The actual measurement is somewhat irrelevant as long as there is some play as you can rest in the knowledge that, in normal use, as things warm up it increases.
You could also try a 14T front sprocket to lower the final drive ratio which will negate the need to slip the clutch so much when riding slowly off road.