Well, I guess we find out tomorrow (Friday) what the resolution to this problem is.
After many emails with HTC, I sent them my phone. When I queried the length of time the phone was in repair, they told me they were waiting on parts.
The phone has been repaired now and is being couriered back to me.
I'll let you all know what I find.
EDIT: Spoke too soon. I've just had an email from HTC saying "Due to the delay in awaiting parts the device has been swapped to avoid any further delay."
So all I can do now is test the replacement when it arrives and see if it suffers the same fault. I'll post all the technical details for comparison as they may have a more up to date firmware, radio, etc on the replacement. It better not have an S3 though!