I wouldn't trust em! 
I moved into a house with a prepayment meter. Gas company said they'd reset the meter and send a card. They did. Every time I topped up it took last tenants debt and left me with no gas. After 4 top ups being snatched by the meter I refused to put in any more no matter how many times they promised a refund and told me the meter was now reset. Each time it wasn't and it took my money. I left them after a week and switched companies. I then got a bill for £170 for the 7 days usage I'd had. With no gas as the meter just took the debt but didn't give me any gas.
Bloody Scottish Power cowboys.
Keep a record of your calls to them in case they don't fix the problem. Write down everything they tell you. Take names and heck, record it if you can. It took me 6 months + to get Scottish Power to rectify their multiple errors. Having my short hand transcripts of the conversations I had with their employees helped as I could tell them exactly what I had been told in previous conversations.
It's funny but be cautious just in case.