Slight misfire - plugs?

[Gnasher]

Checked the connection, sprayed with WD40, checked again, still not working properly, level might be low though. Problem is that I've checked the expansion tank today and it is empty.

I'm a bit confused here you mention fuel gauge, now on to coolant in the same sentence? 

give it a good run whilst watching coolant level.

You can't ride while watching the coolant expansion tank is next to the top of the shock on the righthand side of the bike!

It might be that there was air in the system and it is now properly filled with coolant (well, better than it was), will add coolant to the expansion tank and see if it settles or is leaking somewhere.

Yes it's possible the coolant level is often on or below the minimum mark as a rule. The cooling system on the Fazer is totally reliable if filled with the correct mix even with almost no maintenance.  I my experience changing coolant is not necessary just test the gravity, if low add as required.  I've never changed any coolant in any bike I've owned or worked on just test, even when doing major engine work collect and refill.  The fact that coolant is not the usual bright green mix tells me the engine has had issue! 
         

Fazer as leak (well, empty expansion tank) was spotted day before the test.

The expansion tank level is normally quite low, check the pipe from the system filling cap which is under the tank.