The Scot signed a three-year deal when he joined the Rams last summer from Preston but, after guiding them to promotion via the play-offs, has put an end to speculation over his future by extending the deal by 12 months.
Chairman Peter Gadsby said: "Over the last few weeks we have been in discussions with Billy over his contract and we´re delighted to announce he has agreed an extension to 2010. That is very good news and I would say it is like another new signing."
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