Idle when at running temperature should be 850-900 RPM.

Turn all electrical equipment off and then with the engine running, disconnect the ISCV connector, this is behind the hose coming from the header tank in your picture.

This shouldn't make any difference in speed, you need to undo the locknut on the T/B, then tighten the screw which will lower the speed, when you get the above figure tighten the locknut.

Reconnect the ISCV, at which point the speed should momentarily increase and then drop again.

You are lucky to find a standard example!

Martin