how can is top my sparkplugs arcing?

[Dead Eye]

Yep, definitely sounds like the thread for plug 3 has been re-tapped incorrectly / out of spec

Might be possible to have it redone but I'm not sure if that's a sensible approach? A second hand head is about £50~ as far as I am aware (based on other threads). The work required to replace it is fairly straight forward as it can be done with the engine still in the frame. A little time consuming in that you will need to make sure the engine timing is correct on reassembly but I'd expect a competent mechanic could do it in 2-3 hours? Complete guess though

Could take a bash at it yourself, timing the engine isn't too difficult on the Fazer from what I've heard - I will be doing this when I get round to re-assembling the one I have in the garage