Sorry I'd not seen this post sooner! I wrote a literature review when I was at university about this. All I could find from my notes is a 2002 Oscar Faber "Croydon Tramlink Impact Study" that estimated the transfer from car to Tram as 19% - 16% as the car driver and 3% as a car passenger.
I think you will find such an actual shift is quite limited, much more will come from buses (especially to New Addington), plus generated journeys. Contrary to much belief, journeys in urban areas by car and by public transport are commonly not readily interchangeable.