There are lots of equations to deal with when buying a used car. Availability of Auto Trader divided by newsagents open at 7am on a Friday, multiplied by cups of tea in a cafe, for instance.

Or phone calls to shifty blokes who will only agree to meet you in a car park divided by how many examples of the car you're after are available on that day. That one never works in your favour.

But it's the technical equations that really get me going: equipment levels expressed as a fraction of the cost when new, engine size plus bhp plus fancy interior kit or, my favourite, list price divided by massive depreciation.

And there's little that depreciates as much as, or is as well equipped as, a top-end BMW, which is where the M535i comes in. Once this was the quickest car BMW made (140mph, 0-60mph in 7.4sec), and cost £17,950 new.