Bosch Worcester Greenstar Boiler - No Running Correctly After Draining Down System

[DEL BOY]

I recently drained down a friends heating system to enable the removal of 4 radiators for decorating purposes.They have a BOSCH WORCESTER GREENSTAR BOILER on refilling the system today I found that I was unable to get the boiler to run for more than about 3 minutes before it locked out.After turning the temperature control to reset the same thing constantly happened. I believe that I should vent the boiler by removing the casing and operating the vent. Am I correct please.
Thanks in advance for any advice.

[DIYorNot]

Do not open the case to the boiler unless you are Gas Safe Registered or know what you are doing.

What pressure (bar) did you re-pressurise the system to?

Did you vent the system after re-pressurising?

Did you add inhibitor to the system to replace the amount that was lost during drain down?

[DEL BOY]

I am not Gas Safe Registered and therefore will not be removing the cover. I am an electrician and on reading the boiler manual it talks about bleeding the boiler.
I have spoken to the installer now and he says, I quote :- that I will not have drained the boiler and suspects that the pump is not working. I believe I can check this by removing the large screw on the front of the pump to see if it is spinning. I have checked that it is getting an electrical supply.
Regardfs.