Balance transfer credit cards – spending money and not being charged interest

As many of you I’m sure also do, I have two credit cards opened entirely to transfer large balances, taking advantage of 0% interest for X months offers, in order to provide me with additional short term liquidity.

I don’t normally spend at all on such cards. However, I have begun to do so on one such, which is a Sainsbury’s Bank card, because it is advantageous when Sainsbury’s or Argos is the merchant.

Unfortunately, I no longer have a 0% interest offer on new spending, so I must pay off *new spending* in full each month. The vast majority of the balance, however, is a balance transfer that still benefits from 0% interest, and so I don’t want to pay that part off.

I therefore want my Direct Debit set neither to the minimum amount (as it usually would be), as that’ll now generate interest; nor to pay the full amount, because I don’t want to; I want to pay the minimum on the balance transfer element, and the full amount of the new spending element.

Is there any automated way that I can do this, or is it a matter of leaving the DD on minimum, and then manually making an additional payment each month once statement is generated?

Any tips very gratefully received, as I had not anticipated this monthly inconvenience.