Sold the BMW on. The further I dug into the timing chain issue the more horror stories I found so I decided to get rid.
I looked at the Mazda 6 2.2D but (having done the research *first* this time) found similar issues with timing chains. It looks like a lot of modern diesels with timing chains suffer premature failure because the DPF regen cycle allows fuel to contaminate the oil, which then doesn't lubricate the chain properly, allowing it to become stretched and fail.
So, now I'm looking at spending a bit more for an efficient petrol model instead. Currently no idea what to get though.