The important bit in Bruce's detailed response is: "a round box with a knob on top which one can turn to vary the output voltage". The rest is just to show of the wealth of his knowledge acquired as a result of his age
Variacs can be extremely useful for roasting (I know that Phil used to make extensive use of one, not sure if he still does). I'd run some tests first though to see how consistent your roasts are, before considering investing in one.
Personally Geoff, I'd send Steve an email, as he will likely be able to clarify what is a legal requirement and what is a practical concern. I've spoken with him about this kind of stuff in the past and it never ceases to amaze me, the extent to which we are "nanny'ed" by ruling bodies, and supposedly for our own good!
The main issue for most of us around here is going to be the risk of "baking" beans (as mentioned above) due to overly long roast times. If you were thinking of using a long, cheap, extension lead to roast in a draughty garage during the winter, then that would be best avoided! If you only want to add on a metre or two (to what is an unfortunately short lead (due to EU regulations apparently)) in order to position your roaster under an extraction hood, then I think you should be fine. I did find that my roast times were extended if I had my breadmaker running at the same time (but that may just be the old wiring in my house).