The small claims court will hope you have sent them a LBA, before going on to making a claim with them - it gives the company a nudge to first settle before court.
Agreed, I should have clarified that I meant that the OP tell the company that they will lodge a case if they do not arrange collection and a full refund, rather than automatically lodging case.
Oh, and send the letter as recorded delivery and email them them once you have proof that the letter was signed for.
You (the OP) have documented proof that they delivered the wrong product. If they charged you for delivery- you are absolutely not liable for the return costs and are entitled to a full refund. I would expect the court to find in your favour in the blink of an eye.
The last time we had to lodge a case, it was with a local plumbers' merchant (we purchased a basin, returned it, was told there might be a handling fee, 4 months later we hadn't had any refund- they kept fobbing us off). We turned up in court, they didn't bother. I was there as a witness. The judge listened to what we had to say, because they didn't bother turning up he did not hesitate to find in our favour. My GF was able to claim an addition £75 for my loss of earnings as a witness and the cost of lodging the case. We got our money back pretty quickly. The firm now has a "satisfied" CCJ, which probably harmed their credit rating nevertheless.
BTW, the judges are not scary, it is pretty informal. It will be you, the judge, a clerk, and possibly someone from gravelmaster sitting at a table.