It was indeed air trapped in the pump which upon opening the large screw ⅓ turn released the air!

Boiler now fires correctly.

Thank you so much for the support and assistance.  

Thank you for the help.
It was the none return valve sticking (filling loop) on the mains water inlet.  Turned on water and gave valve a sharp tap with a spanner and 'whoosh' water flowed and pressure restored. Will advise BG of valve sticking issue when they service system or tomorrow as it's covered under my system care plan!

But the pain continues  . With 1 bar in the system cold and all rads bleed of air the boiler one only fires for about 20 seconds then turns off never to come on again until it's turned off at the mains some 20 minutes passes then rety and again fires for 20 or so seconds etc..
I am having BG call tomorrow (Thursday) to investigate further.... Oh the cost!!!  

Good afternoon chaps.

I have had a leak on my central heating pipework. Had to drain down and replace a foot or so of pipe on an elbow!  

None of the draining down or repairers caused me any real issues (other than taking the floor up to access the pipe!). I cant check the repairs are 100% as yet due to the problem below.

The system is a sealed pressure one - Megaflow HW tank, two motorised valves - one for CH and one for HW tank. But the problem is I cant get any water pressure to show on the boilers pressure gauge and there seems little water or pressure in any of the CH pipework going to the rads. Even though the inlet filling loop tap has been on and then off and on again. In fact no mains pressure water can be heard passing though the filling loop! Normally you can hear the water flowing! Other than for about two seconds initially then it stopped!  Inlet filling loop is in the upstairs airing cupboard by the Megaflow tank with the two motorised valves and the boiler being below it in the kitchen.

I'm beginning to feel I have an air lock?  Would draining down and restarting the filling process again remove the air lock?  

My method if you would check I'm right is...   Boiler and all controls are turned off at the mains power switch.  Check the filling loop is Off (which it is normally), open both drain down taps (Flow and return) and then open air bleed screws in all upstairs rads then once water flow slows, open all downstairs air valves, all the rad valves are fully opened during this procedure.  

My method of refilling is.. Close both drain down taps, close all rad air bleed valves then turn on the filling loop. Normally the pressure shows on the boiler (about 1bar) and I can commence bleeding air out of all the ground floor rads then do all the upper floor rads letting in more water as the pressure falls etc. and normally its job done.  

But I'm totally stuck at the moment as there is no pressure and it appears no mains water going into the system.   Help!!!  Any suggestions to the cause and how to overcome it much appreciated.
Only thing I haven't done is move the motor valves?  CH is a BGMVP-2 and the HW tank is a Drayton.