After adding mj12 to robots.txt the spate of bots died down and finally ceased, at least on the web pages (I assume robots.txt is still hit occasionally).
I also now notice an absence of "fake" mj12 bots which prompts me to wonder:
a) were they really fake or simply out of date and out of control; OR
b) did the drivers of the fakes simple lose interest; OR
c) were the bots themselves gradually updated by the fakers (who in that case were running genuine mj12's).
I doubt very much the mj12's were (eg) renamed nutch bots or we would still be seeing them
Credit to Majestic for obeying robots.txt. Nevertheless, I still will not permit distributed bots on my server. There is far too little control over them OR too much hassle setting up for them (eg mj12's validation).