We had a similar situation with EDP a couple of months ago. Got a bill for €480+ euros and since we pay by direct debit they took the money from our account. This is our new house which only has builders there and we know the meter had only gone from 3811 when we bought in March to 4024 when we got that bill.
Anyway, after much time on the phone it turns out that they considered the starting meter reading to be 98865 (we did actually recieve a bill to this effect, but the meter reading was printed on the back in tiny letters so I didn't notice it - and it *definitely* wasn't that). So we had to log a complaint and start all that process which has taken weeks.
A couple of weeks ago we got a new starting bill with the correct (3811) meter reading, so it seems that our complaint has been upheld. The previous owner has been on the phone to us in tears because they're chasing her for the money now. Friday they refunded *some* of the money, €276.41. Maybe that's the refund less the meter reading now, we've been using the furo a lot, but I need to check. I'd written it off tbh.
Anyway, in future I'll be checking bills very carefully!