An update on my situation.
My white HTC one mini developed a number of cracks in the places that everyone else has reported (on the back under the beats logo, in the thin strip near the top on the back, around the volume buttons, near the headphone jack, plus a few more).
I went back to the T-Mobile shop where I bought it and they were not very helpful. They said they could send it to the repair centre. I was expecting them to change it on the spot. Oh well.
So I phoned HTC directly instead. After explaining the problem they said they were unaware of this issue (which I find very hard to believe). They said I could send it in for repair BUT if they found no manufacturing fault with the device itself and deemed the cracks to have occurred through misuse then I would have to pay for the repair and the carriage!!
I was going to leave it and just live with the cracks, but then I thought NO, as I am positive I have never dropped or done anything else bad with the phone.
It's been away for 1 week and I just got it back. They didn't replace the phone, but have completely replaced the white plastic bezel. I didn't get billed for anything.
So all good now.
I will be watching the device very carefully from this point on to see if/when I get any cracks appearing!