Thanks for the input Freddy, I'll give the iridium plugs a go next time I change these ones. I forgot to update this thread about the resistor plugs because Christmas and beer drinking seemed to get in the way of things.
Well after fitting the non resistor plugs as suggested by Den and not the resistor plugs recommended by the Haynes manual, my bike starts quite a bit easier than it did before and the motor seems a lot smoother, but I have only tested it on the short side road outside my house due to it not having MOT or Tax. I'll have to take it for a long run to fully see any substantial improvement but already I can see some benefit to having resistor plug caps and non resistor plugs.