Hi guy's. I've a wee update for you.
So I stripped the old coils off and removed the spark plugs. Coil and plug 3 were in a shitty condition. Rust and that white furring all over. The plugs aren't that old, as in changed the service before last (two years and maybe 6000 miles), but the coils look original.
I ordered new iridium plugs and sourced a set of coils in good condition (with the bracket, fuel pump, filter and hoses) from a 17,000 mile bike. My bike is 42,000 so if I get half of the 25,000 difference from them I'll be delighted. I connected everything back up and took it round the block and it seems fine.
The weather is good tomorrow so I'm going to take it for a proper run up to Tyndrum or something. I'm confident that the problem is sorted though, so thank you again foccers. Another Fazer brought back to life 😊 👍