I wrote to Jenna at the beginning of November 2012, and I got a reply on 16th February 2013. Used the address:
Jenna-Louise Coleman
Troika
10A Christina Street
London, EC2A 4PA
UK

I wrote to Alec Newman in November 2012 about a charity auction, and got a reply on 15th March 2013!
The address I used was:
Alec Newman
Markham, Froggatt & Irwin
4 Windmill Street
London W1T 2HZ
UK

Pictures of the cast cards he sent be can be viewed on @WaterlooRoad06 's twitter