Thank you for the help.
I couldn't get the overide to work but I dowloaded the entire Joomla installation folders/files, then made a copy of the Languages folder and subsequently the mod_footer txt file.
In that text file I made the required changes and first tried it as an overide in the langauges folder - that didn't work.
I removed it from there & put it in the languages > en-GB folder ...... works a treat.
My solution may be a bit clunky but I now know how to do it and should the file change on updates, I can easily go back into the server to reinstate the changed file. I don't anticipate having to change many such files, therefore whilst it is something to always remember following an update, it is not too much of a chore.
Thx again for your help.