Thanks for that,I have tried getting it down with the idle adjustment but I'll check the butterfly,makes sense now you mention it.Might get a chance to do it now before the next bout of rain comes in....Check that the throttle butterfly closes & isn't stuck slightly more open than it should be.
On the flapper (no stepper motor) it is correctly set when you can get a 0.05mm(0.002in) feeler gauge between the butterfly & the bottom of its bore.
To adjust the idle speed remove its anti-tamper plug & turn the idle adjustment screw on the plenum chamber (Clockwise decreases speed - anti clockwise increases speed)