Sorry to necro this, I just want to agree with the others, as the best BMW specialists my family have been using for a few years is Mark Purcell.
He looked after my dads E46 M3 Cab for about 4 years and managed to sort out a lot of problems the previous owner had neglected to get sorted out. He also did some work on my mum factory JCW Mini, when it wasn't going to be covered by the BMW/Mini Warranty.
When the M3 required some big jobs doing, they were perfectly fine with my dad (which meant ended up being me) sourcing some of the parts ourselves and they checked them over, before installing them on the car. They also have a really good bodyshop attached to them, so they are able to deal with pretty much any problems you might have.
Mark is located in the Horsepath Trading Estate opposite the Mini factory. The businesses full address "The Hangar Horton Road, Stanton St John, Oxford OX33 1AG"
I plan on using Mark Purcell to look after my Z4M-Roadster for however long I keep it/my folks and I remain in Oxford.
Back in 2007 to 2011, I owned an e30 Alpina and I had tried a few of the BMW specialists dotted about Oxford and I would always end up having things that were not quite right, or they just really didn't want to be dealing with an old e30 and didn't hide their distaste with me.
If you have a Mini that is covered with the BMW care package, the Mini dealership connected with North Oxford are really good. If you are able, ask to see the service manager by the name of Becki. I've know her and her partner socially and she looked after my dad really well, whilst my mums JCW Mini was being done with them. This is before Becki realised it was my parents car, as she didn't realise how rare my family surname is. She really knows her stuff and is really good at getting stuff all organised, so there is as little negative impact as possible when you need to take your Mini in for work on it.