OP - mental health issues have a devastating impact on finances - I would say that 70-80 per cent of the people I used to deal with at work had depression or mental health issues. It affects how people judge and manage finances, again moneysavingexpert has a massive guide on this.
Mention it to CAB - there is nothing to be embarrassed about, dealing with this, as well as depression makes you more of a warrior than you realise. You will find strength you didn't realise you had. The CAB can refer you to a law centre, if your mental health problems were bad enough, that your capacity when you took out the credit to understand the contract could be questioned. Again this depends on your exact circumstances so mention it.
Mummyofalittedragon - the cynical logic behind it was that companies would rather take one pound payments than nothing, and never hear from the customer until it is time to launch legal proceedings. But once an arrangement like that goes in, it is very hard with nationwide to have any credit with them again, cards are frozen and then accounts closed in those kind of serious circumstances.
My other advice OP is when going through a budget and listing your expenditure, be realistic - don't say to them or the CAB, you can survive on say 40 a week for food if you cant - because if your plan isn't realistic, it wont be accepted. I have looked at income and expenditures before, and although it looks good when outgoings are minimal and repayments are slightly higher, I would think there is no way that will work and the customer will stick to it. It looks like bullshit on paper designed to buy time, rather than a serious attempt to tackle debt. Be honest. I would also keep a spending diary for a week, in a little notepad, that accounts for every single penny that leaves your hand or bank account. Bus fare, clothes for DS, nappies, baby wipes, any medicine like calpol for him, prescription charges for you - things you forget will show up and that will help you.
Be kind to yourself, it takes a lot of courage to admit you have a problem, and a warrior like spirit to tackle it, but you can and will do it, but put yourself and your son first, be single minded, your sons welfare, your welfare, your finances and waaay down the list after the neighbours cat, your husband (cause letting you starve in the day shows he doesn't give a rats backside about you lovely) x