Hi Peter, I'm not sure how much experience your team has with running user testing, but sometimes a little help from a pure expert goes a long way. When it comes to usabiilty, sometimes a little knowledge hurts a lot because there's so much you can inadvertently say or do that primes the test participant or otherwise distorts true behavior. This often leads to the wrong observations and leads you in entirely the wrong direction. I've seen some horrible outcomes from user testing.
Even if you hire someone for 1-2 hours to look over your approach and/or script, it can make all the difference. Again, I don't know the background of your team and you may have some HCI wizards, but I thought I'd share just in case this wasn't the case.
Good luck with whatever you decide!