For what it's worth, I have been keeping a close eye on the Plus 8 market over the last 8 or so months. I swapped my plus 8 for an Aero S1 last year and then recently sold that to pay for some unexpected house renovations etc. I'm desperate to get back into a plus 8 when funds allow and therefor keep a close eye on values.
I can honestly say that the values of all Plus 8's increased quite dramatically over the last couple of years but those increases have now stopped and indeed, fallen back a bit.
There is, as has been said, a glut of them on the market and very few are selling. This is a reflection (IMHO)of the current state of the economy and overpricing the car in the first place.
As an example, Brands Hatch Morgan have a really nice Supercharged Plus 8 and have been trying to sell it for the owner for 10 months. In that time it has reduced in asking price by about £4-5K and they still havent sold it. I'd say about 15% to 20% has come off general asking prices in the last 8 months for these cars.
Other dealers have also reduced the asking price for their stock and this is the case accross the whole market I think.
It's not unique to Morgans, try selling a 996 Porsche!!
Your car is very very nice but the market has shrunk and if you want to sell, either hang on for a long time and hope the right buyer comes along, or review your price.