The stats are pretty low really - Google won't plot graphs of reCaptcha passes and fails because they're too low - but they seem to come in multiples of five.
There are a surprising number of bots that reach but don't even make it past the signup page, because of a clever trick which I won't disclose but which will be obvious to the trained eye ;-)
Further to that, when I get an evening to work on the damn thing I plan on integrating the signup form with https://stopforumspam.com/ which while I wouldn't use it to actually deny someone an account without human intervention might be pretty good for "quarantining" suspect accounts.
One thing to watch is that a lot of folk have signed up but never completed their registration, possibly because they use a Hotmail or Live.com account which filters spam pretty aggressively and doesn't like the confirmation email. If I see someone who looks "real" I'll check their username and email on stopforumspam and then fire them an email from my personal gmail account, which usually has better luck with spam filters.
If anyone felt a bit underemployed and fancied demonstrating their Python skills, I can point you at the github repository :-D
R-reg 4.6 HSE on LPG
R-reg 4.0 SE on LPG
F-reg Citroen CX 22TRS that has now been sold :-/