Just moved into a house where there is prepayment meters. Never used them before. Gas is on em cred but not using any at the mo. Elec was showing zero so pressed and got £5 em credit. Put £15 on the key and inserted it and the £15 flashed up for a second then disappeared. now showing £1.85 like it was before... where has it gone? can previous owner have had a debt on it?