Ok, two things, if it were the spare wheel knocking, it's quite a different noise, been a few mates cars, and picked it immediately. Just an annoying knock. There's a large bolt that goes through the wheel, and has a plastic hat on top to protect the plastic tray thingee under the boot carpet.
If it's the driveshaft (we're talking the long carbon fibre cylinder (not the axle assemblies you lot refer to as driveshaft's ), anyway, the driveshaft universal joint can become excessively notchy and will put out a huge amount of vibration throughout the car, through the seats, firewall etc and will be quite audible. Maybe off base here, but these are from personal experience going by what you've said.
Anyway, you've escaped that one. Keep looking, if you're worried about other Zeds you look at, ask some kind soul from the forum that lives nearby to come check the car out with you. Or buy one from the forum, you stand a much better chance of buying a pearl.