Spraying Problems

I've just painted my new bulkhead, which is my first time using a compressor and spray gun in anger, and I've been having problems. I've got a Clarke AP14GF gravity feed HVLP gun and whatever I try the maximum fan pattern is pretty small and it doesn't seem to be putting enough paint on at once, which made spraying a bulkhead pretty tedious, although it did mean that I didn't get any runs...

The paint (from Paintman) had 10% thinners added, if not slightly more.

The compressor is a Clarke Boxer 14/100 which as the name suggests can provide 14cfm from a 100 litre tank, which from what I've read should be more than adequate. I had the regulator set so that it was pushing through 40psi when the trigger was pulled. The air throughput did seem to be quite high, which made spraying smaller items like the vent flaps a bit interesting as they swung around and needing holding in place.

Any advice would be welcome.

The figure you give for your compressor is its air displacement.
Makes small compressors look much more capable than they really are.
The important bit for compressors is 'Free air delivery' which is the volume of usable air it delivers.
This is usually around 1/2 to 2/3 of the displacement figure.
That said, I would expect yours to be capable of running that gun.

What bore size hose are you using? It it isn't a full bore hose (min 3/8" or 9mm bore) then you are severely restricting the volume of air arriving at the gun. The curly plastic hoses are particularly unsuitable.

Do you have the regulator at the gun (mini one mounted on the gun inlet) or at the compressor?
If at the compressor you aren't able to accurately control the pressure at the gun & there will be a significant drop at the gun.

As far as setting your gun up is concerned I've had a look at the instructions & would make the following suggestions.
With the gun empty unscrew the paint flow adjustment knob (8) then pull the trigger fully & screw the knob back in until you feel it stop.
Unscrew the airflow adjustment knob (6) fully to its stop.
Adjust the spray pattern knob (9) to give an even fan of paint.

Be worthwhile watching some of the 'how tos' on youtube.
This is a basic setup how to: