Cheers, have installed that now, belt and braces and all that (in fact that bot blocker acts before my hack does... ho hum).
Interesting though - the Xrumer software must have been manually programmed to automatically answer all of our human verification questions because since I've enabled the bot blocker plugin to block any registrations that take less than 15 seconds, it has been working (ie the bot is answering our HV questions so must be programmed since the questions are impossible to answer by a bot (well, ours are anyway due to the way they are worded in colloquialism and such that a bot would be unable to answer)).
Well... that or the HV system has a hole in it.