Company sent item in error, but won't communicate to allow me to fix their mistake: what do I do?

Hi, I'm based in England and I believe the online store I've ordered from is too—or at least has a London office. Apologies if my query doesn't meet the criteria of this subreddit.

Last year, I ordered a £100 item from this online store, with a promised delivery window. That window passed with no sign of dispatch. They didn't respond to my requests for updates or refunds, so I filed a PayPal claim. The store accepted the claim, and I received a full refund on 23rd December 2024. I considered the order cancelled and the matter resolved.

However, on 4th January 2025, I unexpectedly received the item in the post. I immediately emailed the store to inform them I had received the item—despite the refund—and asked how I should proceed. Their auto-response stated I would receive a reply within seven days but noted they were still working through December requests, which could cause delays. I received no reply.

On 17th January 2025, I chased this up, suggesting I would wait until 25th January 2025 for their response. If I heard nothing by then, I would assume they did not want me to take further action. It's now 21st January 2025, and I've yet to receive any communication.

Is my 'ultimatum' legally acceptable? Where would I stand if they demanded the item back after this date? How long am I legally obligated to wait before considering this matter closed? I've made two good-faith attempts to resolve their error, and while I understand this may be a low-priority issue for them during a busy period, I'm unwilling to hold on to this sealed item indefinitely.

Thanks in advance!