Starting issue. Seeking help/ advice

Hello all,

I have a GSR600 which started to sound a little too much like old farm machinery for an inline four. It was sounding like at least one cylinder on the left hand side of the engine was struggling (loudly), especially in wet weather. It was also struggling to hold higher speeds for prolonged periods (riding on the motorway etc). I thought I'd start by checking the plugs and coils which all seemed to be functional however I did find traces of oil on the plugs on that left side of the engine (they weren't soaked in oil but as you can see a definite residue). This oily residue plus the fact that the problem seems to be worse in wet weather makes me think it could be the valve gasket that needs replacing but would be interested in hearing any thoughts.

However the starting issue mentioned has occurred since replacing the plugs, coils and airbox. The bike started just fine forehand but now is turning over but not starting and the red 'fl' light is on. Clearly there is something I have done/ not done but I'm wracking my brain to try and determine what. All help/ advice/ ideas gratefully received.