OK so I've a Light and Motion HiD lamp with a NiMh which is about 6 months old.  The battery is supposed to have a 3 - 3 1/2 hour runtime, however last night it only managed 2 hours from fully recharged.  The site suggest a few rounds discharge/recharge to bring it back to capacity, does anyone have any experience of this working or should I try and get a new battery under warranty.

I doubt that the warranty will cover it unless it's faulty. I had a Cateye Stadium that did the same thing after 9 months. I sold it and got a different light!