Thanks so much for your reply. It's helped a lot already. I'm realising the move to 6 strings requires a bit more attention to my technique than I anticipated. On closer inspection, my wrist angle was definitely the culprit.
I've raised the bass right up (higher than I ever thought I would!), tilted it on a bit more of an angle and just need to keep an eye on my left wrist, altering my playing whenever I see an angle start to appear. It'll probably just take a while for my technique to become second nature. I really need to concentrate on it at the moment.
Unfortunately it seems those big spread chords might be out for me though, as they instantly force my wrist into a sharp angle. Do you have any tips on how to play these without the health risks?
Here's an example of me playing the bass in question, with a chord progression I'm working into a composition: http://youtu.be/r-VuFGxFgU8?t=2m18s
Lastly, I just want to let you know your video on the moveable anchor has been a great help keeping those sympathetic tones under control!
Link for anyone reading: http://www.youtube.com/watch?v=hDzRqeS0ruQ