Replaced Coolant Outlet and now there's no hot air..... Bbrrrrrrrrrrr!

Hi there.

I replaced a severely leaking coolant outlet yesterday on my 2006 TDV6, which as you can see from the picture, failed where the two halves join. It was pictured after I easily pulled it apart by hand.

The job itself was very straightforward and took only half an hour... after I found out the throttle body simply pulls out from the car after taking out the two retaining bolts, removing the attached hoses and EGR pipes and then disconnecting the electrics.

Now however, after reassembling and refilling the coolant, I have no hot air from the heater.

Slightly concerning is the crumbling of one of the pipes on the old outlet (circled), but I found no bits in the pipe attached so it looks like this has been a gradual deterioration rather than one big chunk coming off.

So, I'm assuming there is air in the system somewhere. Can anyone provide some instructions on how to go about bleeding the system? I've found lots of posts but they mostly refer to the petrol variants and I'm not sure if the diesel has a different cooling/heating setup?

Thanks in advance.