Hi danny, ive got exactly the same problem.
I`m putting it down to pressure on the underside of my arm from the edge of the desk as i`m using the mouse.
I`ve applied a support bandage which is helping a little.
I agree with most of the posts, as I suffer from RSI, caused by to much playstation, good few years ago, which every now and then comes on while at the keyboard! But any numbness, especially in the arm, I would check out with my G.P. �Hope This Helps!
i agree that it could be caused by your arm or elbow resting on the edge of a desk or table while your typing cos i get exactly the same thing and i think its caused by the nerves in your arm being compressed.