candy white ED30's are rare and sought after so you would expect 20k with private sale, then go armed with the cash.
You are having a laugh, right? OK, white is realtively rare and therefore may be worth a
little more than another Ed 30.
£20K in a private sale

I saw a very high spec Ed 30, 5-Door, DSG, Xenons etc etc. in a VW Dealer, 08 plate for less than that. OK it wasn't white but I guess you're a little optimistic.
Based on the book price £17K is not a bad offer at all. If you could sell it for more than that in a private sale, you'd be doing quite well...