I've checked rails, mounts and barrel. All tight.
I've gone back to Sightron and demanded that they take another look at the scope before they send it back.
To be fair, John at Aim Field Sports is taking this problem seriously but I get the impression Sightron in the USA aren't very interested at all and its them who have to correct any faults with the scope.
Walther LG300 Dominator
Sightron III 10-50x60
Air Arms TX200
Big Nikko 10-50x60