Actually not so strange once I thought it through carefully.
In our translations we often embed variables for things like number counts. For example, there is a phrase “Total individuals: xxx” where xxx is a number. The translatable text is therefore WT_I18N::translate(‘Total individuals: %s’, $count_indis) . Here the result of the calculated variable $count_indis is inserted in the text at the position of ‘%s’. Doing it like this ensures any language can work with the same variable even if its position in the sentence needs to change. But it is essential that ALL languages include the ‘%s” variable. In the en_GB file the ‘%s’ variable had been accidentally missed from the translation. Hence the error message “Too few arguments”. It was looking for the variable in the translation and it wan’t there.