Yeah and pre-payment, according to Scottish Power, doesn't cover all costs.
I recently moved home (again, private landlord bastards), and was confronted with an almost £50 charge for gas, even though I was on pre-payment. When I enquired how and why, I was told that pre-payment doesn't actually mean that as any extra charges will be added on when either moving house or changing supplier. Extra charges meaning if I used emergency credit, which sometimes was unavoidable but it was automatically paid back when I topped up the meter, and somehow, VAT.
I refused to pay this extra amount so obviously they stated with the threat of debt collectors calling.
No wonder everything is in an anonymous call centre somewhere