And at a guess the change the OP refers to happened years ago. Correct, when they added 1s in all the codes. (Still, ours hasn't lasted as long as my aunts. That was three digits 60 years ago. It's 11 digits now, but the last three haven't changed.)

Made me think for a moment as UK number can only be 10, 9 or 7 digits long ... but add the 0 to make 11!

Have you tried inputing just house number and post code into the bt checker instead of your number, if you have done,what results does that give you ?

The BT adsl checker has been for the past 2mths been giving different results for my line, none are correct though,

Entering address does indeed show the sensible result that ties in with what my ADSL reality used to be: Between 10.0Mb and 19.5Mb (Estimated speed: 17.0Mb).
Also gives sensible figure for my neighbour based on his telephone line.

As I said at the start, I'm not too concerned, just intrigued by the inconsistency.

when I had my line installed the adsl estimated went down sharply, After FTTC was activated it went back to close to what it was although still a bit lower, a slight concern but only slight because I have no reason to go back to adsl at this address now.