Bought my car 4 weeks ago and the PTU went up the creek but the car was driven about 500 miles or so with a misefire causing the cat light to come on etc etc anyway to cut a long story short its now fixed :D would the misfiring cylinder/s have caused damage to the spark plug/s concerned

Others may know better but I would expect that the cylinder that wasnt firing would just have run cool and the unburnt fuel would have been burning in your cat making the light come on.

 

The cat would probably survive and hopefully the fuel in the cylinder wasnt so rich as to wash the oil off the cylinder bores increasing bore wear. Still it was only 500 miles.

 

The spark plug or plugs affected might have got fouled a bit with unburnt fuel deposits but if she's running fine now it means any deposits got burned off.