Thanks for the return.
I am 'sync'ed to my Ancestry.com tree, however this quirk was evident before then.
It occurs when I accept data from followed hints on Ancestry.com. I don't think it is Ancestry.com or FTM that has done this tree. It may be some other database software that has converted the place name to some sort of index code.
Some avenues I have pursued and rejected...
Grid reference x
Google map reference x
Soundex code x
If you do an Internet search for a particular code, then very occasionally I can match a code to a place.
1671160, Middlesex, England = Hampton
1671403, Middlesex, England = Wood Green