The cable keeps slipping out of the cable guide fin and pulling tight, resulting in a complete inability to change gear.

Has the STW hivemind seen this before? Is it a design flaw? Would putting a zip-tie round it fix it / be a good idea?

Cheers

Have you fed the cable through the hole in the guide?

What coatsey said. Otherwise it's probably snagging somewhere in the outer (like where it might have been cut).

Not sure which hole you mean. Where on the guide is it?

It's half way round the cam and yes I have heared of people having trouble with this and yes using a cable tie to fix.