airtronics are the same as sanwa, find out which model it is in the UK and then find the parts you need according to the servodatabase.com its the ERG VR over here
http://www.servodatabase.com/servo/airtronics/94357z
give SMD a ring and find out if they have them in stock, im going to go out on a limb here and say they sell them as a gear set only though.
or find someone who has just fried the electronics side of things who could sell you the rest
which once I have checked at home which model I did kill ill let you know.