So I have had an email from the TTB complaints department acknowledging the problem with wifi channels changing on their own when set manually. They said they will notify the Product Department and a firmware fix should be pushed out in due course.
Also heard from Sagemcom Support who I contacted about a firmware update but they essentially fobbed me off with saying the correct route is via ISP to report to Sagemcom who would issue a firmware to TT and it would be pushed by TT.
talktalkbusiness.net & freenetname
TTB Router, Asus RT-AC68U and ZyXEL VMG1312-B10A Bridge on 80/20 Meg Fibre
Highest Sync: 79993/19661