Hi Tony, I have only just joined the Forum, so apologies for the late reply. I had an E89-owning friend (recently moved away) that up until the end of last year was actually using Sytner (ne Specialist Cars) Stevenage for his work. They have a separate (large) facility near to their showroom, and I seem to recall they had (have?) a discounted parts price tariff for previous gen vehicles. He said that they always did a great job and were actually competitive with other specialists once travel cost/time were taken into account.
Not sure how the change to Sytner has impacted things (if at all), but they may be worth consideration. I got my Z4 M40i from those guys and they were great to deal with, and I used them for servicing work on my previous M4CP, where they always managed to discount below my local dealer in Bishops Stortford, despite the emergence of ubiquitous region-wide 'menu pricing', and were always very efficient.
I'm not far from you BTW - you can tell from my user name I guess...