Thanks Justin.
You're right that it's not very user-friendly, although I managed to input all the correct digits from the bill - they are all down at the bottom just above the barcode.
It didn't work, and asked for the barcode too, which still didn't work so I've failed to get it to work.
There is an e-mail address on the bill, so I've sent them an e-mail [using online translation service] to ask how to pay. The guy in the office had said it could be done by phone, but the number he gave is now unobtainable.
E-mail address if anyone else wants it is manilvaclientes@agbar.net
That's for my area of course and other areas will differ.......