After the chat that I had posted about previously, I filled one of the customary surveys and made known how I felt about the service from that chat.
Someone from the HTC complaints department rang me yesterday to get more details. I explained the situation to him. He sounded like he was trying to help, and in what I thought was a fairly courteous and professional manner explained that HTC were not able to deal with this issue directly with me, but they had processes in place with the distributors and retailers which they did deal with.
He said my comments would be fed back to the head office, so I made known my concerns on this issue, my main point being that a normal user would be very unlikely to discover this issue within their retailer's 2-4 week return period (if they even had one, the UK tends to be quite consumer-friendly in this respect), and therefore HTC needed to be able to do something about this.
For me, I'm conversing via e-mail with my retailer now (I bought the phone online and their physical premises are hundreds of miles away in another town). Hopefully they'll be able to take it back. HTC have my IMEI from the ticket and have probably just marked my handset as unwarrantied at their end so flashing the RUU won't solve that problem.