you need a solicitor
at a guess, letting agent has been paying the bills upto december, then stopped paying them from when the landlord took over. might simply be that the landlord doesnt know the letting agents had done this, so he hasnt been paying the bills, since they were not in their name / address and they probably havent even seen them
how often did you / letting agent check meter readings to calculate who owes what, and have you been paying your share and who to? same for upstairs, have they been paying their share to letting agent/other?
you dont want a pre pay meter - these will simply charge you more for electric and are used to clear old debt, which isnt yours. what may be an option is to take out a new contract in your name, and either come to an agreement with upstairs or simply turn off their power otherwise