If the seller is a private seller he does not have to accept returns or cancellations for change of mind.
You have paid so probably the best thing to do is to wait till after the delivery date and then open a "I have not received it case" from the item in your summary. Escalate after 3 days and you should get a refund. That does not cost the seller anything "£40 indeed!"
[I am assuming seller does not send item]. If he does send it and is a business seller you need to open a "return" and pay the return post. It all must be done through a case; every time.
It sounds trite perhaps but eBay is designed to be used to buy things that you actually want. The facility for change of mind is limited.
@leahf123