A standing order is different from a direct debit in that you would have needed to set it up with your on bank directly for you to pay the money out. A direct debit is set up by the company receiving the money with your permission for them to take the money.
As your husband would have needed to have set up the standing order, his bank can give him the details of who he pays it to if they have them,
There are very few companies now or even then that would be taking payment by standing order, most use direct debit and then they have control over taking the money. There is a risk with a standing order that the payee can stop it at any time.
I would speak to his bank again and ask them for the original standing order authorisation.
In the meantime, think through what he could possibly pay annually for that amount that he doesn't already pay. Is it an annual premium insurance policy - did it start at the same time as your mortgage (or a year later as he may have pasid an initial premium by cheque) What about AA cover? House insurance, Dentalplan, Pet insurance,maintenance cover for an appliance, annual travel insurance, payment to a factor for the upkeep of shared gardens, bin cleaning, subscription to a magazine, charity donation etc etc.