Thanks for the comments guys.
I did think of a S2000 but for my budget I'm still looking at quite an old model.
The reason for comparing the civic and the 350z is for the same amount of money I'd get a newer type R (07/08) compared to a 05/06 350z, or a 03/04 S2000.
Practicality is way, way down on my list. I don't need boot space, nor do I need more than 2 seats and that's where the 350z has that appeal.
I think the next step is to go for a test drive on the 350z, which should be exciting. I was taken for a ride in one from the Birmingham NEC (can't remember the charity, but you pay £XX and get a ride on a wide variety of cars). and it was pretty awesome. That soundtrack. Hnnnggg. But who knows.
I'm still quite open though. I've been in a few remapped Diesels which have been excellent. Sure it doesn't have a decent soundtrack but having that torque and reasonable power whilst maintaining 45mpg does have an appeal of its own.