Lack of hot air when heating is on full-blast?

I've been a busy bee! 

So I've discovered I've had a coolant leak, originating from the water pump, which I've gotten around to replacing last week. Whilst I was doing this I also have the cooling system a flush out as well, including the heater matrix inside the cabin.

However I was unable to remove both hoses from the bulk head, I was able to get the lower one off (which I believe runs to the water pump?) and a load of rusty water came running out  .

After filling it back up and letting it idle on the drive for a while, I was able to get some heating back! however this was short lived as after a 20 minute drive it appears to have gone cold again.

Any idea what could be the problem? I've had a go at seeing if there's any air trapped in the system this morning buy putting it up on ramps and "burping" the coolant pipes but so far that doesn't seem to have worked?

I'VE FIXED IT!!!! 

Or rather bided some time until summer comes back around and I can do more work on the car.

A very helpful post on a facebook group indicated that I should try some "percussive maintenance" underneath the dashboard where the heater matrix is.

I used a mallet to "gently" bash it and the blowers started pumping out some warm air!!! (or much warmer than it was anyway)

I'll give the cooling system another flush with a cleaner of some description this weekend, hopefully that'll help further.

Also in the process I snapped the small arm off that connects to the bowden cable on the temperature controls. Could any one point me in the right direction of where to get a replacement? TIA 

Hi did hitting it work or the flushing? I have the same problem as well as the car not coming up to temperature? have changed the thermostat this year but still not up to temp have brought another one to try hope this works any ideas would be great.

So It worked briefly when I hit it, but a replacement heater matrix has arrived now so I'm going to swap them out