Well, an update in case its of use to others, so it's not prepay power it's a prepay with Electric Ireland (who, let me say, have been most helpful) and it turns out that when the tenants moved in (they were a couple who since broke up so only one moving out now) they had a previous debt from another property which moved with them into our apartment, where they installed the meter without my knowledge or consent, and in 2.5 years never mentioned it. EI now can't release (give me a deactivation code) until I talk to their Credit Control on Monday and prove that we own the property, not connected to the tenant, etc etc which means in the meantime I've no power to proceed with getting carpets cleaned and all that. Most worrying is how the remaining tenant was not forthcoming with me about this, tried to act dumb, eventually I've been able to clean that the original debt was 900 euro in the ex-girlfriend's name, still is, he is paying it off he tells me but says he doesn't know how much is left!! His deposit was only 570, cos that was the monthly rent - I've been way too generous - which I've had to hold until we get this unknown amount sorted out. Should be straightforward enough but we don't know until we know, and clearly he hasn't been honest with me up to now. On a positive note, EI have been nice to deal with and they say the meter can be removed at no cost once the account situation is sorted out.
I thought this couple were lovely, she is local, I met both her parents, not much more you can do to vet someone. So disappointed, being an accidental landlord is the biggest expense and pain in the neck.