Regretfully, I very much doubt that any of us diesel drivers are going to have any equity at all in the coming months / years, such is the plummeting value of the cars at present. I had a couple of trade in prices on mine back in December (only 7 weeks ago, and both were within a hundred pounds or so of each other), and have had 3 valuations in the last week (just testing the water). Best case scenario, the car has lost £1350 of it's value in that time (worst case nearly £2k) !!!
I think it's something we just have to accept until it's realized that the witch hunt on diesel passenger vehicles (certainly Euro6 models) was and is totally unjustified. I've just decided to really enjoy the car until it's time to part with it and see then what it's worth, and if it's less in part ex than the balloon payment, VWFS just will get the keys handed back to them and I'll walk away.
Who remembers the time only a few short years ago, when buying a VW diesel guaranteed some of the best residuals in the market !