To think this (online) shop should refund me

[mrsfoof]

I bought some things online at the end of May. I paid for first class post and got an email to say the order had been sent the next day. I've just realised that it never arrived so contacted the seller (an online shop, not Amazon / eBay) to ask for a refund.
The seller is refusing to refund because it's been 3 months and this was the first time I'd been in touch. Their reason is because it's too late for them to claim from Royal Mail for the lost parcel. That seems unfair - it's not my fault they can't claim anymore and I've not got the stuff I ordered.
Any ideas? Can I take them to court (seems a bit extreme as the order value was only £50 but I can't afford to lose this)? Annoyingly I didn't pay with PayPal.

It's not a question of what would be reasonable

You are wrong. That's precisely what my question was. It's a valid question. It's a straightforward question. I'm not asking about the likelihood of proving it in court.

Would it be reasonable, after nearly six years, to get in touch with the seller and say 'oh, by the way, I never got my widget that I ordered. Can I have a refund or a replacement please?'