Hi Andrew, i have a 306 XLDT, when i changed the battery it happened to me! (I had to lock and unlock using the key fob) it could one of two things? The starting motor or the key fob? Does it have a deadlock button on the key? If so just lock it using the key fob then unlock then start the car and this should sort your problem, if its the starting motor put it into 2nd gear and rock the car back and forwards, then try to start? try this several times, (its better with two of you) Other than that it will have to go to the garage. Hope this helps?
Lee from Bournemouth, January 2009