Just bought this today from Sissinghurst in Kent but it didn't have a label and we are just deciding where to plant it - whether it likes full sun or not.

It's a Dahlia of some kind. Yes, full sun.
It's root is a tuber so it will grow back next year.

It's a Dahlia.
In England, Roger, you will need to lift the tuber in the autumn and save it in a cool, frost free location this winter for next year planting.

Thanks for the info folks