I think the new ones retail for between £600 and £700, so this Ekstreme Chair by Terje Ekstrom seems quite a bargain.
Ekstrom originally designed it back in 1972, but the chair didn't go into production by Stokke (now Varier) until the 1980s. As much a piece of sculpture as a seat, it's got a metal frame with a cushioned cloth covering and by all accounts is very comfortable.
This example is described as being on 'good condition', with some stitching to the main seat. So not perfect, but at £100, priced well.