yes they can! i have had so many problems with working tax/child tax credits. your working tax credits were paid on what you earned in the year previous. you can ask them for time to pay etc and they are pretty good in that way. you will have to repay it without doubt, if you think that you shouldnt have to repay it(although your question states you had it) your local mp will probably reluctantly help you out.
if you dont get any joy through the tax credit system phone your local tax office and rather than speak over the phone get an appointment to go and see someone and explain to them.
they will help, although somethimes not how you want them to.
hope it helps