They probably can, but on the basis that the error is theirs, they should be reasonable about giving you time to make up the shortfall.
Tell them you'll increase your payments by £10 per month until the shortfall clears. If they query that, say that you are demonstrating intention to pay, and re-advise them that the error is theirs.
Keep copies of everything, and send your letters to them Recorded Delivery and keep your receipts safe you need to fight further up the line.