What are these Thonet chairs worth?

From the label and the fact that the company only moved to York in the 70s I assume they're not very old. Can anyone tell me whether they are original Thonets and are they worth anything? I don't know whether to try to sell them as is (though it looks like they have already been re-upholstered) or whether I should sand, varnish and re-uphoster them to look more unique. Any advice would be greatly appreciated. Thanks.