Phil is dead right you need a ball which is a sliding fit in the hole. I suggest you take the tap into your local bike shop. Chambers are brilliant but this sort of thing is expensive from them - especially with p&p.
Any spring that fits will do although for positive indexing - but it does need to be quite strong. It doesn't protrude too far from the hole (otherwise it would get coil bound when compressed as the tap is rotated).
If you continue to have problems, let me know and I'll strip my MkV to get the dimensions of ball and spring.
Happy Shooting!! Paul.
"We cannot solve our problems with the same thinking that we used when we created them" - Albert Einstein.