Thanks for the replies & tips guys
I've already tried the battery off for 20 minutes to reset the PCM back to factory settings, but it didn't work
Regards autobox fluid level....That was the first thing I checked (up to temp, on level ground, in neutral) & if anything, it was a little high. Had to syringe half a litre out to get it to the right level, but it didn't make any difference.
It only holds 1st gear too long after you first start it (hot or cold) & after that, it is fine.
One thought had occurred to me.....that maybe the transmission may have had a partial oil, filter & gasket change at some point. The service history is a little vague in what jobs have actually been carried out over the years & the trans fluid looks a little too clean for a vehicle with 87K miles on the clock. Could it be possible that it's been filled with Dexron III rather than the recommended ATF+4 (not so widely available in the UK).
I've read that ATF+4 is the recommended fluid for this box as it's fully synthetic & has a friction modifier. Dexron III is part synthetic & doesn't have a friction modifier.
I'm not convinced that it is a fluid problem though, because as I said before, it only does it the once when first pulling away after starting from hot or cold...after that it's fine. It's as though it's programmed to do this at every start up whether hot or cold. I'm starting to wonder if Lornaben may have a point in that there is a dealer software update for my Jeep
Thanks again for all the assistance folks, best regards, Jon