SMI - attribution of payments when two payments fall within the same month

I have been trying to find anything in the regulations which gives guidance as to how SMI payments are to be attributed to the client’s mortgage account. My client has had 2 payments in April and his mortgage company have attributed one payment to the contractual amount for April and put the other payment towards reducing the arrears.
He is up to date with his court order which requires that each month the contractual amount is met plus £20 otwards the arrears. Is there any legal basis where we can argue that this second payment should not have been used to reduce the arrears ? I have read righ through ESA Regs Schedul 6 but did not find anything here.

Soc Sec (Claims and Payment) regs, looks to be schedule 9A(4).

We came across something similar, with a certain (northerly) bank attributing an SMI payment to the “unsecured part of the mortgage” in preference to the mortgage (apparently their terminology for “a separate and unsecured loan we gave you” ). We wrote to them quoting an appropriate bit of the regs, I’m afraid I can’t recall the outcome.