Heels: Under the Distance Selling Regulations (which are in the law of all EU states), one is entitled not only to return a defective or incorrectly-supplied item for a full refund of the price paid but also to have all outward and return shipping costs refunded. The seller cannot contract out of this. So you should insist on a refund of your £6 return cost (and anything paid for the original shipping to you as well). Please update us on your claim.
Early this year, I bought some Birkenstock sandals through Amazon Marketplace from a seller in Germany. Despite a clearly-placed and acknowledged order, I was sent sandals in the wrong size and colour and had to return them (which cost around £15). Their replacements were also the wrong size and colour! Another return was necessary. After a fair amount of argument, involving Amazon as well as its seller, I got all my outlay back. It should not have been necessary for me to have to demonstrate to Amazon its seller's obligations in order to secure my rights. (I was then told by the seller that it had no stock of my required size/colour, although it was still advertising them as available. At this point I found another seller on eBay, also in Germany, and got the correct item at a slightly lower price without any aggravation.)
When I get time, I will post some further general notes on consumer rights when something is defective. Just bear in mind that it is the retailer who is resonsible for the remedy; any warranty (e.g. by the manufacturer) is in addition to one's statutory rights and may be of little or no extra value. A defect appearing within 6 months of purchase will be assumed to have been present at the time of purchase unless the seller can prove otherwise. After this 6 months, the onus is upon the buyer to demonstrate the fault was there at the start. But, that aside, the general rule is that something must last a 'reasonable' time, given its function and cost etc - so don't simply accept that no claim is possible after 6 months.