You're being a tad over analytical over the figures Dave.
For what it's worth,335D prices do indeed seem stronger now than a year ago,but doesn't help if you're trying to sell,analyse that
There have been at least 5 for sale on the forum over the last 4/5mths,and no one seemed in the least interested.
I tried selling mine on AT in Nov last,it was on there for 3 wks,first week @£22,500,the last two wks @£21,500,I received not one call,nor email!
Yes,arguably it was a shite time of yr to sell,but there seems to be far more 335d's on AUC now,then there was back in Nov last year.
Your car is worth far,far more than the CAP valuation of £21k,that's laughable,and it's worth at least £25/26k retail,so at the moment you are in equity,and by some margin.
My old car,an Oct 2008 model,with 33k mls,and a very similar spec to the one you have,is up for sale on AUC for £22,994!
Two hopes for achieving that though,one of those hopes is 'no'.
Thankfully I came away with a nice chunk of equity,and it goes towards my fighting fund.