Okay did some more driving and I am going to go out on a limb with this one, as they say I am a big boy and prepared to take it on the chin if I am wrong.
You need 8mm ring spanner and 2.5mm allan key and you need to undo the lock nut on the throttle body then screw the screw in with the allan key, Do it when the engine is hot raise the revs to about 1400rpm (the ECu will pull it back over time). Lock the nut and thats it, if after driving for a day you can make further adjustment.
See photo.
Why does this work, well base revs is now controled from the throttle body not for the valve, so when you lift the throttle and the valve cuts the air the throttle maintains a base air supply.
So there it is a 2min fix and so far it works perfect for me. As far as I am aware the SVX is the only car I have ever owned that has fully closed throttles every other one had it slighly open.
Good luck and I hope it works for you all. If it does we will all kick ourself for not figuring it out sooner.
Have a great day.
Tony