Hello Andrew,
I assume that what you mean is that you have a number of payments for various supplies that show in the bank feed as a single amount.
Have you considered using the Prepare Electronic Payments feature of MYOB?
While this works best when you send an ABA file to the bank , you can also use it for preparing multi B-Pay and pay anyone transactions, by putting a dummy bank details on the supplier card, recording & saving the aba file. But you do not send this to the bank.
By doing this when the Bank feed comes in with the multi payment you can match it to the single grouped MYOB transaction.
Have a look at this MYOB Support Note on Preparing Electronic Payments.
http://myobaustralia.custhelp.com/app/answers/detail/a_id/9244/kw/9244
http://help.myob.com/wiki/display/ar/Electronic+payments
Cheers
David