No defence whatsoever. It it ALWAYS the responsibility of the rider / driver to remain within the limits. Speeding is not a misreading, forgot to look or blame it on faulty equipment, it's exceeding the limit regardless of what ANY user device may tell you. Riding with no, or a knowingly defective speedo is also an offence so I wouldn't 'mention when questioned'.
I do find it highly unbelieveable in that OK, 45mph is not a great deal faster than 30 - but in an urban setting it's pretty obvious as you pass every other vehicle.
By extension, 'thinking' he would be doing 110kph (70mph ish), when in fact he would be doing 110MPH is just as dubious a defence.
He may however have a civil case with the last owner - but probably only if it was an authorised motor trader. Private sales are normally subject to "You bought it, YOU should have checked the merchandise - it's yours."
ps. I'm not being unsympathetic - just maybe a bit too blunt!! No offence intended
