Alasdair, I've done a little more than read a book about it. I worked in flight test instrumentation for many many years. That is where I found out mass balance does little or nothing to prevent flutter. To prevent flutter, you must change the modulus of elasticity, mechanically change the coupling forces, stiffen or lighten the surfaces, remove the stimulus plus many other factors. Again, mass balance alone (unless you change one of the other factors in constructing the mass balance) does NOTHING to eliminate flutter. Check with a good airodynamict for more info.