Probably about £15.5k I'd say.
Having just sold and Elise, the Zed is much more liveable car. Yes, the Elise was HUGE fun, but I had bigger grins doing things right in the Zed. With car prices the way they are, I would predict you taking a hit on the Zed whereas Elise prices are doing very well. Summertime, economical and low tax....