tax credit.HELP!!!

You can find the Inland Revenue leaflets giving more information about tax credits here A Guide to Child Tax Credit and Working Tax Credit

but there is absolutely no reason why someone on tax credits shouldn't also have a mortgage.

My only reservation would be that many people with tax credits regard this as a reliable source of income which can be depended on absolutely. Unfortunately there have been a number of overpayments and many people have found repaying such overpayments places them in an extremely difficult position. So particularly if your find you are given a higher tax credit payment than you expected it is worth double checking that the credits you are getting are exactly right and ensure that should there be any changes in your circumstances, these are notified to the IR promptly and you retain evidence that you have done this.