Of course the assumption (or is it presumption?) is correct. The whole damn thread is about tunefix isn't it (it being the thread in this case)
Right then, to try to be a little more specific. Prior to installing tunefix, I had channel del and renumber running. Renumber reallocated LCNs 1, 2, 3, 4 and 7 to BBC1 HD, BBC2 HD, ITV HD, Ch4 HD and BBC3 HD respectively and put the SD versions of these LCNs at 101, 102 etc.
I uninstalled renumber, but did not delete any associated database. I uninstalled channeldel and its associated database as suggested by prpr in post #4. I then installed tunefix and left the settings at default, as I noticed that it was going to re allocate BBC1 HD, BBC2HD, ITV HD and Ch4 HD to LCNs 1, 2, 3 and 4. I did not investigate where it might put the SD versions of these LCNs.
Having done that, I did a full automatic retune. On completion of this, I found the SD versions mentioned in my previous paragraph at LCNs 101, 102, 103, 104 and found BBC3 SD at LCN 107. I then checked the lower LCNs and forund, as expected, BBC1 HD at LCN 1 etc, but with BBC3 HD at LCN 7.
The reason for my query at post #68 that raised the 'what is it' question, was because I hadn't specifically set tunefix to do this (put BBC3 HD at LCN 7). As it (whatever that might be) has put BBC3 HD at LCN 7, I really can't be assed to try to find out why, but was just asking in case there was a quick answer.