OK. Time to fall on my sword.
Here it is, straight from the horses mouth, checked, double checked and triple checked (with abject apologies for the forgoing misleading information.)
It is M5, and those of you who said as much can enjoy the bragging rights, whilst I wipe the egg off my face.
That is to say, (to cover my backside, and also to be as transparent as possible to everyone's interpretation on thread specification and nomenclature) the diameter is 5mm and the pitch is 0.8mm
(the thread length is 10mm)
Yay!

