I seller is a UK registered business seller they are required by law to accept returns for items purchased online. If they refuse, have a word with trading standards (if possible, in the seller's area). They may be able to help.
As you requested the return straight away is shouldn't matter if the legal returns window in now closed.
Provided it was paid for online I think this still applies, even if you collected it in person - but not if you paid on collection. Trading standards will know whether this is correct.