I have just found out that my brother inlaw has power of attorney over my Father,he no longer wants him to have this how do we get it back?

If your father is competent to make decisions, he can execute a document revoking the power of attorney.  You would want to have that revocation delivered to your brother in law, and any third party whom you believe he has shown the power of attorney to - for example, if he used the power of attorney to open an account at a bank in your father's name, you would want to present a copy of the revocation to the bank so they will know that the power of attorney is no good.  It would be wise to document the date and time the notice of revocation was presented to each entity - having a signed receipt is usually a good idea, but consult with a local lawyer about what they suggest.