Hi Beamer boy and welcome along.
Lots of people recommend ac cars but their prices have crept up to within shouting distance of the main dealers so questionable if you will save alot that way.
MKW motors in Kingswood business park gets some recommendations here.
When you say always been to the dealer does that mean it has five years of dealer stamped servicing? This will be an addition when you are selling a premium car, most of the indys will now stamp the service book, also there is the option of buying the filters from the main dealers, keeping the receipts and getting an indy to fit them. If the car has five years of dealer servicing and you are thinking of selling it soon might be worth getting one more done - sorry prob not what you wanted to hear. (on my last car I decided at the start if I was going to go all main dealer servicing or none)
Obviously your 530 is a high level car and most enthusiasts recommend going to a specialist, but I have always found that the good independents do a good job and I like that you can go in and chat to guy that is actually working on the car.
I suppose it depends what category you fall into