Stelrad NF50, doesnt stay on. rads hot then go cold!

[andywade2002gy]

The Make of my Boiler is: Stelrad Ideal nf50
nf50 natural gas: 116782A01

Problem: boiler starts from cold, heats radiators up until all hot, then after 20 minutes it stops and will not come back on again leaving the radiators to go cold.

it wont fire again until all radiators freezing cold?

pump (set to level 2)
port valve (mid position)
control panel (set to run CH only)
room thermostat. (set on highest setting)

 

[John506]

Where is your Room Thermostat situated and what temperature is it set at?

 

[andywade2002gy]

room thermostat is under the stairs in the hallway.. set to 30 degrees but heating is not working naturally..