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:

That's really helpful, thanks. I'll give it a try. The hose is a fairly substantial rubber one, not sure of diameter but it's not one of the coil jobs, and I've used euro fittings throughout to give a better flow. I forgot to mention that I've got an inline filter between the hose and the spray gun to remove water, so I'll try taking that off to see if it makes a difference. The regulator is at the compressor end.

Datasheet will give a rough starting pressure for your paint. 2.7 bar seems pretty high for a hvlp, but the datasheet will tell you.

You need to set the pressure to give the atomization that you want, and then adjust the needle on the gun to give the flow.

A longer hose from the compressor to the regulator / trap and then a short hose to the gun is a sensible setup. Make sure the air flow control in the handle of the gun is on max.

Lots of people have a gauge on the gun too, but, if you've just got a short hose between the water trap and the gun, you won't get much pressure drop.

You do need the datasheet (TDS) for any of the materials you are spraying as they give correct mixture, fluid tip size, air pressure, flash off time between coats etc.

I too have a moisture separator & the hose to the gun runs from that.
You don't want a sudden squirt of water arriving on the paint & I would suggest you leave it in place - I'm assuming it's a full size one, not one of the mini in-line things which aren't going to help with airflow.
Don't forget to drain the compressor tank, surprising how quickly water can build up.

All my guns - SATA, Devilbiss & Iwata, full size & SMART repair size & all gravity fed HVLP - have a gauge on the gun inlet so I can be sure the pressure at the gun is correct.
They're not expensive & you can be certain that pressure at the gun is at the gun manufacturer's spec. Anything else is guesswork & introduces another uncertainty into the equation.
Some of the expensive top of the range guns have the gauge inbuilt.
Check & adjust the pressure with the trigger pulled so you have air flow through the gun & then final check spraying paint. If you put the gun down then recheck before starting to spray again.