WooHoo! Found some on Venom! £4.84 each! Not bad at all Powerflex ones.
Will have to ring and check they are the right ones but they look right!
Cheers Nick! 
I had the same problem and tried these but they stretched just like the others. Mine gets a bit of abuse round the track though.
In the end I used the GSF ones and also put a large cable tie around them. I've had no problems since.
The magnex exhaust is about 7 kgs lighter than the standard exhaust so god knows how long they would last with the heavier standard exhaust. Another good design by VW!
Cheers
Paul