Interesting stuff. I have the same problem, at the same speed (50) under braking. Thought it was down to wheel balancing, and was putting that off until I get the new tyres I desperately need. But if that were the real reason then it would surely wobble at 50 whether braking or not? Sounds like I need to replace the discs - and maybe start using the handbrake more... 
Where's the best place to get this sort of work done? This is my first privately-owned car after many years of company (Vauxhall) driving - and I vowed I would never go near the local Vx main dealer if it were my own car. Any recommendations in MK/Northants area?
Baz.
Well since your up the road m8 from my regular home, i could do it for a small fee

Serisouly though its very satisfing to do jobs yourself, you also know its been done right! Ask for some quotes but i did mine for £66 + vat, this was for GM Discs and Pads (you get a set of pads with the Disc set)
I changed mine with guidence from TheBoy on his driveway, took about an hour per side. But this was my first time, so would be the same again. I do have photos of me doing it, and was going to make it into a guide but never got the chance. Think i'll post them up anyway for reference.