Looking at that diagram, Autosparks, there's only the one indicator relay? I've got a Mk4 with the i/s indicators sometimes not working outside, while the o/s are fine.
I'm 98% certain the problem is a relay, rather than a fuse (would work or not), or a loose wire (even when stationary, the problem persists).
Can you shine any light?
Also, hello. I'm new here.