Nah ive just had a better look and it appears the thread inside the nuts is flogged out. Maybe the bolts werent done up tight enough in the past letting them move and vibrate around and flog out the thread?

Ok so a 19mm spanner fits perfectly (snuggly, no movement at all) onto the nut. That means it i should use the metric bolts i got? Whack em in and crank them past the last two rows of thread that are the problem?