Central heating not working after drain down

[Harvest Fields]

Why did you see that?

 

[tmm]

Why did you see that?

In the Manual

 

[heatservice]

Are there any air vents in the roof space?

 

[tmm]

Cheers everyone for the help Got it sorted.
You's were all spot on with the air in the system. I opened the lowest drain off valve and just let the system flush through for about 10 mins then closed the valve off, I then switched the pump back on and set it to 3 for about 5 mins to flush air through , i switched the pump back off and opened the air vents as shown in the photo above. Switched the pump back on and it all works a treat
Cheers once again