I first noticed this about 2 weeks ago - a mild vibration though the chassis at 60-70 mph; I don't drive it that often and my wife doesn't often reach these speeds (speed limits rather than capability  ).   Last weekend I drove it again and it was more noticeable and worse at 70 mph, fading away at (ahem) higher speed.   Today I took it to Kwik-Fit to get the rear wheels balanced - they did this (small weights were needed) but the problem got worse, now a serious judder at 55-60 mph.  They