where is overall_footer.html in a phpbb installation?

I have an old phpbb installation which has lost it's statcounter connection sometime without me noticing. Probably on an upgrade.
Statcounter says to insert the code in 'overall_footer.html' but I can't find that anywhere.

Any clues where it is?
Well you need to find wherever that is located. Check your phpbb documentation and see where you can customize the footer of your current phpbb installation.
I have an old phpBB establishment which has lost it's stat counter association at some point without me seeing, presumably on an overhaul, stat counter says to embed the code in generally speaking footer...
No idea, but you must find the particular php module which produces the </body> that is used for every page. You add the tracking code there, above </body> . You need to use an ftp program.