I'm with Dan on the email thing. It protects you but can also, by the clever use of words, be positioned as just a helpful confirmation of what you think you are going to be doing. I know this sound odd but try and keep the tone of them as friendly as possible, so that they are not so evidently the backside covering things that they actually are - and make sure you get a response back - i.e. yes that's right - objective, timescale, cost, any interim reporting required, etc, Doesn't need to be long.