Hi
Just to add I was with TalkTalk up to a few weeks ago and never had those gaps.
ADSL and VDSL is a best effort service, with all sorts of hacks and tricks to try and work around the problems of sending high bit-rate data over wires never designed for it, and all these sorts of oddities are just part and parcel of that unfortunately.
The missing tones may be added to allow gaps for other lines pilot tones, or something TalkTalk has added on some equipment to disable those tones for a reason only known to them. BeThere did something similar which was always a cause of bafflement to many why they did it, as it prevented some from joining the 24Mbit club
Even if you identify the cause of the missing tones, I doubt very much you could ever fix or change it to get them back.
Regards
Phil