Is it on the same side as your 'mouse arm'?
yep, I'm right handed.
Also, do you crane your neck forward e.g. looking at the keyboard or documents flat on your desk?
Fairly good touch typer so don't really need to look down at my keyboard. No documents on my desk (well, a notepad, but I'm not constantly looking at it). Monitor straight in front of me, about 40cm away from eyes, set up as high as it will go with the my eyelevel being about 1/3 of the way from the top of the screen (if this makes sense)?
Does a stretch like this help (with chin tucked in)
Yep, does relieve it for a bit (works better if I stick my right hand behind my back and use the left to pull it across and down whilst bending my neck to the left, with chin tucked in)
You got a health & safety bod in your company? If so get them to do a workstation and posture analysis on you.
Unfortunately not, we're a pretty small business... Here's a pic of my rather basic setup 
IMAG0407.jpg by ten_sim, on Flickr
Have you recently upped your Vitamin-P consumption?
I have indeed, do you think it's somehow related? 
