A: possibly yes. (Never thought I'd find myself saying that :-))
Took out a CW storage tank and HW cylinder yesterday and found a magnetic gizmo on the 15mm copper feed to the system. The storage tank had a very small amount of scale encrustation around the water level line. The cylinder had no evidence of _any_ limescale in the bottom (it was as light as a new one when I lifted it out (not quite one-handed, but not the usual hernia-inducing lugging/dragging one usually has to do).
This is in a hard-as-nails water area (Reading) where storage tanks often have sheets of limescale around the sides and bottom, and cylinders have half a builder's bag of limescale in the bottom.
Interestingly the last few litres of water (tipping the cylinder over to get out the liquid below the bottom inlet tapping) were a sort of brown sludge.
Someone had written on the magnetic gizmo that it was installed in 1993.
So there you go. I won't be fitting them to my installations: I need something that provably works, and for all I know there could be some completely different phenomenon at work here[1], or it may not work everywhere, and I can't afford to take chances with customers' installations.
But I'd be interested to know if others have similar - or contradictory - evidence. FWIW this unit was called an AQUAMAG and consists of two plastic blocks about 200mm long which clip together over a 15mm pipe, and (judging from the way a ferrous object is attracted to the blocks, and the two half-blocks attract and repel each other) seems to have 4 sets of magnets arranged thus
ASCII art
----------------------- | N----S----N----S----N |
-------------------------------- p i p e
-------------------------------- | S----N----S----N----S | -----------------------
[1] and no, there wasn't a water softener in the system, or any evidence of a bag-in-the-tank type scale inhibitor in the tank.