Thanks again for your replies, had a read up on ignition and resistors and it appears the spark can be made to last longer by using resistors and this helps burn the fuel more effectively. Apparently, the fuel/air mix can stop burning as the piston goes down and compression drops after the initial bang near tdc. There's an issue with interference without them too which can play havoc with ecu's and telly's etc. The resistor spark plugs are usually 5k ohm , same as the caps which makes 10k and a clean burning engine.
My only problem now is I've seen the nice red champion caps too and can't decide what to get