I think given your circustances I would go directly to Natwest.
Write a letter explaining your position, your monthly outgoings, and work out a realistic surplus income, from this derive the amount you can afford to pay back to them over each month. It's in their interest to consider this. Don't juggle credit cards, try and pay of as much as you can on the highest interest cards first.
Because of increasing bankruptcy, many major banks are now willing to tailor your repayments over time, possibly give a number of months reduce payments etc. They would rather do this than go the IVA route were the debt will just get sold on.
Make sure you speak to Natwests dept mangement services. Natwest may well introduce you to Payplan, if they can't help, in which case since they have over 75% of the total dept owed by you, this may be a solution.
Take some time to consider your next step and get as much advice as you can from the proffessionals on this forum. Your at the right place for advice.
Chinease proverb: - 'I was angered, for I had no shoes, then I met a man who had no feet'