Now that I'm home I'll illustrate my previous comment, Airfix would have you glue part C1 to the fuselage half after you have glued the cockpit tub in place.
The reason for this is, which is vaguely illustrated in the instruction, is that it protrudes slightly past part C51, the rear cockpit frame, making it impossible to install the cockpit after part 50 is glued to the fuselage. The solution is to glue part C1 to the tub instead of the fuselage half, as a test fit will show it's resting on the tub anyway.
Also, I forget exactly the conversation but I believe in regards to some query or other on BritModeler it seems that although the color call out for C1 is HU56 it's supposedly a matt black so not sure if you can see it but that's what I painted it.
Here is a good shot of the fit of the upper cowling after I'd shimmed it up,
despite the gaps at the rear you don't want it any farther back as then it starts to interfere with the fit of the canopy, and it actually looks worse in the pic then it actually was. Notice the altitude of the pilot.......
Can't remember if I gave it a lick of Mr.Surfacer or not, but after a splash of color it all looks quite presentable to my eye
Hope this helps a bit