As above.
During summer it obviously hasn't been heating. now the weather is colder, and I've noticed that the Climate system is not blowing any warm air once the engine has warmed up.

What are the Common problems with this? (First thing to check)

I'll run a climate diagnostic on the system today on my way home.

Check the heater tap vacuum hose, probably easier to do this from under the car.

Hey I have the same problem. Where do I find the heater tap?

check ur thermostat...mine was gone and it stopped the heater working

Before pullin stuff apart make sure its not the blend door servo (E2) This can be checked by running the climate contol reset and if there is a problem the E2 may appear. To do this turn the ignition to "on" so as the climate control panel lights up (engine NOT running) hold down the "mode" and "off" button at the same time on the climate control panel. The display then should fully light up and go through a test/reset sequence. At the end if all is good it will show "off" if there is an error it will show "E" followed by a number.

I did the self check and got no problems. I don't have a clue where any of this stuff is... where is the heater tap in my engine?? What do I look for?

Your heater tap is at the back of the engine bay on the passenger side. A small black valve with four plastic pipes and a metal cylinder about the size of a small electic motor.

Unlikely that the vacumn hose is the problem as the default position for the tap is open (ie no vac = open tap), although the tpa itself may be seized shut.

Try manually manipulating the valve a few times and it should tell you if there is a problem there. If this does not work, try pulling the vac hose off altogether as constant vac to the tap will have it shut all the time (this would indicate a problem with your vacumn dist servo's).

Otherwise the next port of call is the blend door motor, although unlikely to be this as the self test comes back ok.

I would then be checking for a blocked heater core or similar.