Precedent? Clearing of backpayment of spousal supp.

I wondered if someone could help me please, I am seeking advice on an existing court precedence in order for the court to support my request.

My ex and I separated 5 years ago and divorced 4 years ago. I agreed 50/50 split of my salary to her but as we did not use a lawyer (I know) I failed to have this coined correctly as a Global agreement. This was of course intended to cover the cost of the children as well as her.

5 years later, after getting in debt, the ex has gone through proceedings to request child maint from me after advice from a debt service. This is on top of the 50% of my salary, which is crippling me. I have a new partner, new house and two new babies. It is unsustainable and unreasonable.

I have paid the child maint. and 50% of the spousal. She has taken me to court for 6 months back payment of the spousal I have not paid her (and her costs of £5k). I have told the court I cannot pay what I do not have, and it is simply ridiculous to expect me to go to work and give my ex 75% of my salary.

The court have advised they are inclined to do a variation going forward but cannot clear the pack-payments of spousal owed, because they don't know of a court precedent where this has happened before.

My question is: does anyone know of a court precedence where this has happened please? This would be seriously beneficial to my case.

Thanks in advance!