Result! Swapped the front to the rears and vice versa, and it solved the problem... the car's as straight as an arrow now

Thanks all for the advice!
One hell of a tedious job, as I cleaned and waxed the inside of the rims while I was at it (if any of my neighbours saw me they'd be sending for the men in white coats!). Was worth the effort though, this steering pull was annoying me. Moral of the story, replace tyres in pairs!
Damn monzas are so easy to mark though... managed to scuff one of them a small bit with the wrench whilst removing the nuts, just inside one of the "holes". Hardly noticeable I guess!