New Pump fitted, but now i've no heating

The water pump on my oil burner was leaking. I changed the seals but the leak was still there. Turns out it was the stop valve on either side of the water pump. I replaced these "brass" fittings, fitted a brand new water pump i had, and no more leak. Hurray.

Now i lost a serious amount of water during this repair. I had the valves turned off but had to turn them back on again each time to check the repair status. IOW was it leaking with water running through it. Which means each time i disconnected the pipes/valves tot he water pump i lost all the water in the pipes.

Once the repair was done and i was sure there was no more leaking i turned the heating system back on but the rads don't heat. The water pump was running fine but made a bit of a screaming noise once or twice and the pipes clunked for a few minutes. I turned the power off immediately and went "rooting".

The heating system header tank had about 3 inches of water in it and it was the ballcock that stopped it. I held the ballcock down and filled the small, heating system, header tank to just below the run off pipe. After an hour it still hadn't dropped even a quarter an inch. So the water coming from the header tank is obviously not making its way to the boiler.

I don't know if its a blocked pipe or air in the system, but i'm pretty sure the boiler is empty. I've contacted an expert but it's looking like over a week or more until he might be able to get to me.

The annoying part is before i fixed the leak the heating system was running perfectly. the reason for the fix was to stop the small bit of water (possibly a litre or so) that was drip out from the damaged valve (on the water pump) each time the heating system ran.

That must mean that it's an air lock or blocked pipe. IOW something simple.

I'm a little lost when it comes to plumbing and the pump fix was only possible because it's so straight forward. However i'm not sure how to bleed the boiler tank, if it needs done, do i need specialist gear for it, or if there is a valve on the heating header tank i should be opening or closing to fill the boiler again.

The guy i rang to repair this for me told me it's almost (95% sure) that the boiler is empty and it's a serious airlock He even told me to go to the heating header tank and open the valve to fill the boiler, but the heating header tank has no valve. It's a direct or straight feed (gravity i assume). Meaning it fills the boiler as it [the boiler] needs it and when the header tank fills sufficiently the ballcock stops it overflowing.

My reason for this thread is i hate to call someone out to fix something that will probably take him 5 minutes. Plus i want to understand how to do it for future reference.

So anyone able to shine any light on this, and while my mouth is warm, pictures would be a huge help.

Thanks in advance.