Right, I've taken the screw out of one of mine and can tell you the following about the screw:
Size: M2.5
Thread: Metric Fine (0.35mm pitch - course is 0.45mm)
Total length (including head): 18mm
Now, I have a set of GB7 knockoffs which I was going to raid for parts anyway and I took the screw out of one of those. It is slightly longer at 19.75mm, and has a slightly smaller head, but I put that screw in my genuine GB7 tuner and it tightened up fine - there must be some tolerance built into the design to accept the slightly longer screw.
Here they are side by side (and length difference heavily exaggerated by the macro lens - I measured them both with digital callipers)
If you'd like me to send you this screw, noise me up in a PM and we'll get it sorted. But if you just want to take the info above to find your own screw, then no worries.