Intermittent A/C

[Spookyfish]

I have recently started having trouble with my AC, I have climate control, it's been working fantastic all summer and keeping the car lovely and cold during the 30c heat wave. It now does this funny thing where the AC refuses to kick in when there is bright sunshine. The fans ramp up to full as cabin temp rises but only warm air comes out. In the evening the AC kicks in and comes out ice cold, I have also noticed it sometimes kicks in when driving on a shaded road. I have had it regassed and checked for leaks but all is ok. I have checked for fault codes and am getting some fault codes for the glow plugs and also a fault code for 17070 ECM power relay short to ground (I believe this one feeds power to the glow plugs) about a week or two before the AC problem I also had a fault with my IMF motor. This has since been replaced. I don't know if these faults would all somehow be connected or if its all coincidence. Can anyone shed any light on this?
Thanks

additional:
My evaporator is quite smelly and when the AC kicks in I get a stale odour coming through for a few minutes.
the compressor fan turns on and off when I switch econ button on/off. The large tube coming from the compressor gets cold to touch but still warm air from vents. Even though the compressor and fans are running I don't get the stale odour until the AC finally kicks in and cold air starts coming from the vents.
If anyone has any idea on what could be causing this or knows of any more troubleshooting I can do then please let me know. I do have an eBay vagcom with vcds, so if I knew what I was looking for I can diagnose with that any thanks