Thanks for all the replies and suggestions.
I've read through many posts about these problematic six speed ZF auto transmissions on a number of sites (these trannys are used in a multitude of vehicles). In most cases, the problems seem to be caused by either a faulty torque converter, faulty valve body, clogged filter/s, lack of service (leading to clogged filters), a great deal of towing, or simply not having the correct software programmed. It seems that many of the X5 V-8 models with this transmission suffer these problems (and other problems too). I have another vehicle with the five speed ZF transmission and it is faultless. That said, I've owned that vehicle since new, so know what/how it's been driven, unlike my X5 which I think was used to tow a boat or possibly something else quite heavy judging by the wear on the tow ball.
Irrespective, the transmission does NOT slip, pulls hard and does everything it should do, except for the slow engagement of reverse and harsh shift from 1st to 2nd when cold. I'm hoping that a re-flash of the mechatronics will cure these problems but I would still like to find out if I can do this myself or I need to take it to a BMW dealer to have this done? I hold little faith in the local dealers as I've already spoken to them about this issue and their answer was "you probably need to replace the transmission as they're a non-serviceable part"... WTF?

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08/06 build 4.4i Stirling Grey over black