Lowells refusing to accept part payment agreement

My wifes cousin got into trouble with a credit card and the debt was eventually assigned to Lowells.

This is a credit card taken out after the rule changes regarding requests for copies of the original agreement.

With some help from another forum and a debt advice specialist he agreed a payment plan with a 50% reduction in the outstanding balance. Luckily he has kept the letter which clearly states they accept his proposal for a 50% reduction with payment plan of £25 per month.

The balance was just over £800 reduced to £400.

He has now paid them £425. Based on evidence from his bank statements. He never missed an agreed payment as it was taken by DD.

He wrote to Lowells to request the £25 be returned and for a letter to confirm that he has completed the arrangement.

Lowells wrote back stating he still has £375 to pay.

When he called Lowells to discuss it they said that the agreement had been agreed in error by a member of staff without being 'signed off' by a senior manager and should have been refused. Thus he owes the money.

When he told Lowells that he has the letter confirming the agreement they said that as it was a mistake they have the right to void the arrangement.

My wifes cousin told them he was going to cancel the DD but the person at Lowells said that doing that would mean they would start the court process. He therefore has not cancelled the DD as he does not want to take the risk og having a CCJ as it might affect his employment.

Can Lowells do this?

Surely an agreement such as this is binding?

Any help appreciated.