It is an asset that you should have declared when you applied for UC!
I'm also concerned that you say that you are receiving UC and Working Tax Credits, this is not normally possible and your TC claim should have been closed when you applied for UC.
I would recommend that you seek face to face advice as a matter of urgency, if there is an overpayment, which appears possible based on your post, then it will be recoverable.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems