About a month before christmas I ordered a couple of clothing items from a small British business. They arrived after a longer wait than expected but one of the items was way too big for me. I normally take XL but this XL was more XXL. I emailed the firm who replied quickly that I should send it back and they would swap for a smaller size. I did so but heard nothing for a while so rang them. The guy I spoke to said it had been posted and to let them know if it still hadn’t arrived. Since then I have heard nothing despite ringing and emailing. I thought I would give them the benefit of Christmas doubt and assume they were shut all over the period. So today I thought somebody must definitely be there but again no reply to email or phone message which went to answer machine.
What can I do if they still fail to answer my attempts to communicate?

You do realise that there are still a lot of places closed today?

give it till at least a week back before chasing, postage and couriers etc over christmas are always massively backlogged, especially for smaller companies whos packages are nearly always de-prioritised over Amazon etc.

the trick is, never do any mail order returns until after the Christmas period

and Euans2 is also on point, hardly anyone where i work today.