I need financial advice

I owe several thousand pound on a credit card and the debt was bought by a collections agency. I pay them £10 a month and they have never said it is not enough or not acceptable. Now they have sent me a letter saying I broke the agreement although I am still paying them, and they are demanding the entire amount in one go.

Can they do this? I am really worried. I do not want to fill out an income and expenditure form, and they agreed to this, as I work for a finance company who did not ask me to do a credit check on employing me. I don't want my employer to find out my financial situation.

What can I do? I am worried sick.