Npower customers have yet to claim £20m-worth of refunds for gas bill overpayments.

Last October, npower announced that it would be offering £70m of refunds to customers in regard to overpayments dating to 2007.

Since then, almost 1.2m customers have been paid more than £51m. But 700,000 people have yet to claim their refund and this week energy watchdog Consumer Focus urged them to get a move on.

Christine Farnish, chairwoman of Consumer Focus, said: “It is great news that over £50m has now been put back into the pockets of over a million people by npower. We want to encourage all those consumers still entitled to a refund to get the cash that’s rightfully theirs.

“We urge anyone with a payment letter at home from npower to take it to their local post office now and get their refund. Anyone else who thinks they may be due a refund should call the npower helpline on 0800 975 7938 to see if they are eligible.”

Npower chief executive Volker Beckers said: “We are working closely with Consumer Focus to maximise the number of eligible customers and former customers who claim their payment. We are still seeing redemptions running at more than £1m per week and we hope that those who have unredeemed letters will redeem them soon.”

npower is offering repayments to 1.89m households in total. These customers are likely to have paid for more higher-priced gas units than they expected to in 2007 after the company changed how it charged for these units but did not communicate the changes effectively.

Claim levels vary across the country, with the lowest in London (47%) and highest in South Wales (69%) and the South West (also 69%).

npower has offered payment through a bar-coded letter that can be cashed at any Post Office.