Help Needed

I hope someone out there can help - I have a series 2 TD5 Discovery year 2000 on a w reg Auto. Just recently the Transmission light has been coming on on the dash - at no specific time, distance or motion. I have had it to my local Landrover garage and they say the fluid level is fine and cant find a problem. Has anyone had a similar problem?? And any ideas what could be causing it??

Thanking you in advance Dave