Hi Mel
Not 100% whether the parts will interchange but I think yours may be this type (fleabay# 170986974321 )
According to Chambers guns yours will be the early type action with the one piece tap and an index plate that fits on the other side (does it have a screw in the centre of the tap that holds it together??) if it doesn't have the screw then the one on fleabay may be the one for you..
Also, it's the little plate with the quarter stuck out bit that stops the tap rotating, it could have worn away...
EDIT
Just had a read at the description and it says it's for a mk4 if that's so then I would say that it will fit as chambers list the later type assembly on a mk4 (the listing on eprey may be wrong I susupect )
John