I have not yet answered the question.
You have referred to a payment of £430 a month paying off the debt in a year.
This suggests that there is much less than £8,000 is outstanding
However to an extent that is irrelevant
The debt is a joint and several debt and if your ex cannot pay for any reason then you will have to pay it in full and there is no way around that I am afraid
In fact it is not up to the bank how it is paid - one direct debit two or more.
So long as the payment is received each month then that is all that is required.
With that in mind I would suggest that you start making your payments direct to the bank so that they do not go astray
If he stops paying then you will have to renegotiate payments with the bank.
Whether or not you are legally separated or divorced or simply living apart the position is the same no matter what.
Please ask if you need further details