For your own peace of mind it would be best to get their agreement first, have you called them and advised that you need to update the SFS as your circumstances have changed ?
If you can't get contact/agreement from them, then as the previous poster said I would cancel the DD and pay what you can afford manually.
You have 30 days from the date your mtg payment is due to either pay or reach agreement with them before it's marked with the ICB as missed payment, so cancelling the dd might focus their attention and you can then get something agreed which they can backdate (if they wish) as if the agreement was in place before the payment fell due.
All of this is an emergency measure though to deal with the immediate problem, but of course the long term situation needs to be looked at too