Hey tom !
Theres two things you may need to do,they are both quite difficult unless your competent with spanners and common sense.(no disrespect !)
first,if your idle screw is all the way in,it sounds like the spring in the valve may need a bit more tension.Directly underneath the idle valve screw you will see a circular blob of glue,behind that is another screw that tensions the idle valve,you`ll have to chip all that away,then you`ll find either a plastic or brass screw that will screw in to tension the idle valve,plastics ones are a pain so maybe you might want to go for option 2...which is...
Where your 3 throttle linkages meet the throttle cable(bolted on the plenum)remove the three(8mm headed nuts)take the adjustable linkage off the throttle control and pull on them slowly and try to feel any resistance,you should feel a slight resistance as the butterfly breaks from the closed position,repeat the process for the other two(each one controls 2 cylinders).Now if you look where they connect at the bottom of the throttle bodies you`ll see a screw(throttle stop) withanother 8mm headed lock nut,the screw has a philips screwdriver slot so you can unscrew the stop and then use the locknut to secure it.If you can experiment with the first screw(which is the easiest to get to)try unscrewing it and feel the resistance improve.