Well ... it's sorted! All you guys that suspected the battery, were spot on! Even though the bastard thing was showing 12.7V! Anyway, as I'd just bought a new battery for our lass's car, I decided to hook it up to the bike before I fitted it to her car. Bonny Lights back! Turned over a bit sluggish, though! Stripped and cleaned starter solenoid, starter motor and battery terminals. Cleaned the main earth on the back of the crankcases (could Suzuki have made it any more awkward to get to?), and re-fitted it. Hit the button again, and cranking speed back to normal! @duncan Same symptoms as you, excluding reverse polarity! Battery showing 12.7V. Soon as ignition switched on, voltage drop to .7V recovering to 1.9V! FUBAR'd then! Bearing in mind FBJ's scary new battery experience, what's decent gear, and what's to avoid?
Cheers Oldskooler's!