Treatment of an old credit balance on a supplier statement

I've just started working for a new company and recently received a monthly statement from one of our suppliers. On this statement there is a credit b
alance of £525.00 When I queried this with the supplier, due to the age of the credit balance (It goes back 18 months!) they couldn't explain w hy, apart from that it's a credit on our account. I quick reconciliation m y end and I still can't get to the bottom of it.
What would be the correct treatment in order to "claim" the credit of ? ?525.00? Would it simply be a case of raising an internal credit note and then deducting this from the next invoice(s) due for payment, thus using u p the credit balance?
Many thanks in advance for any advice.
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