The MOT history check I did before purchase showed that an oil leak had been identified at the last test. I checked for any obvious leaks when inspecting the car in the day of purchase but couldn't find anything. However, after sitting in my drive for a few days I found some spots on the drive which led me to the cause. It turns out to be a common problem where the seals in the air charge line no longer seal correctly and allow oil captured in the intake by the crankcase breather system to leak. This will tend to drip from the front right corner of the car just behind the intercooler.
Although it looks like only the upper seal has failed, the seals are only a few pounds each so I will replace them both. Ordered from Cotswold BMW via the BMW Land forum. Parts ordered by forums members receive a discount.