Unresponsive about replacement/refund

I purchased an expensive hover board as a Xmas gift for my son. On Christmas day we opened and it was an out-the-box failure. I immediately informed them and made arrangements for them to collect early January 2019. Understanding that it was holiday season I initially didn't mind the delays but it is now 3 weeks later and still they have not reverted with a replacement. I continuously get the sorry response that they are liaising with the supplier but that is where it ends. I have now requested a full refund but still nothing. Poor quality product was sold to me and the after-sales service is even poorer. I would recommend that buyers take heed of this before they actually transact.