WTC paid into old bank account

I did not qualify for tax credits, but last year my income fell, owing to illness, so when I renewed my claim, I was owed a considerable amount of money, which HMRC paid into the wrong a/c. Nationwide are my old bank, and I left them 6 months ago. Never told HMRC, because I had not been receiving anything.
I am making an arrangement with Nationwide to pay off the overdraft, but they have now frozen my account, and will not let me take my WTC out, which I need, as being off ill I fell behind with my mortgage, and other bills. What are my rights?