Couple of questions:
1) In the INV PREFS currency sign field, did you set the sign by entering this code: & # 163; (spaces suppressed)?
2) Are you using the eg-invoice template from the zip file?
I’ve done both these, set same values as you in custom.config.ini and set en_GB as Language and Locale fields in the INV PREFS. Using this both the default and the eg-invoice templates work correctly.