Not much help I'm afraid but after my old man's experience do not take it to Autobarn in Northampton.
Its a bit of a drive but when I was in Olney I always used BMWorx in Aylesbury.....my Dad now uses them and gives him a good excuse for a drive...he lives in Northants.
Hello buddy even though I live in west london I take mine to BMW N.Oxford.It is family owned and the alpina forum give me the recco.I wait while its done have my free cappucinos and cake.No complaints.