Hi Trooper,
I myself completed an undergrad degree and a Master's degree, not in actuarial science, but in management. I had a search online for info for you and there isn't much of it out there that is recent - on any category to do with graduates and postgraduates being paid differently.
However, from my experience I am a little advanced in position and salary than most of my friends who I studied with during my first degree. I think much of this is down to my personal determination and willingness to drive myself forward, showing my employer(s) I was the person to promote/hire.
As for exact salary, I'm earning on average £3,000-£4,000 more (per annum) than if I had not completed my Master's degree, but I have a lot more debt than if I hadn't. My advice would be not to complete a post-grad degree unless you want to do it for yourself, as you probably won't end up being that much ahead - I believe this is down alot to self-determinantion and self-belief.
Anyhow here are two sources:
http://education.guardian.co.uk/mbas/sto...66,00.html
http://www.devicelink.com/mddi/archive/00/10/001.html
Good luck in your future decisions