The only way Id buy a Plus Four is if I won the lottery in which case I wouldn't need to P/X my 4/4 and I would probably have both cars.
I dont think we need to worry about the market being 'flooded' with Trad Part Exchanges as the new Plus Four wont appeal to many Trad owners who prefer the old car feel and the £20K price hike and modern feel has put the Plus Four in another market sector for many Morgan owners.