...or 2 year old 320d? priced at 15k and 21k milage but exact same spec . we wont do huge milage and both cars are like two peas in pod except for age and mileage .
The budget ins there for both but as it is a second car give me some sound advice.

I would go for the lesser valued car if you are not going to do too much mileage. Bmw's engines are known for being able to do huge mileage. We have a 318 is coupe that we bought at 2 1/2 years old with 42k on the clock for £14k. It now has 65k and is still in good nick but we are having trouble getting £1700 for it. I think what I am saying is if you keep the car for a long time the value will be peanuts anyway so if they are both the same go for the cheaper one.

I go for the 320d 2 year old for 15k . The main depreciation has been taken, but the market is flooded with used Bmw 3 series as they are starting to take over the mondeo and the like as reps cars. I nearly bought a 320 convertible but a garage local to me was full to the brim with ex bmw used rep cars and it put me off. Saying that the are good cars and the diesel engine will last go on and on if serviced correctly and it will be much more easier to sell on than the petrol. Personally I would consider looking for a Merc, lexus or Audi.

my opinion, get a lexus, much nicer car, less common, handles better, way better sound system

If you have full service history for 5 year old and you are not going to do much mileage I would go for that one. 68k isn't that much over the yearly average of 10 - 12 k is it and if you are doing a lot less you should balance out the high mileage in a couple of years.

Diesel is superior (fact!) Go with the 320d. You're lucky, we never even got diesel bimms, except for a few 524tds.