Heating system not working since partial drain to add inhibitor (and refilled)

MTK

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I have a pumped, gas heating system that was installed c1989 in a domestic, two-storey home.
The boiler is a Potterton Netaheat Electronic.
The pump is a Grundfos Selectric, marked "Type UPS 15-50 130". P/No.59525600.
There is a cistern ('expansion tank') for the heating system in the attic.
In addition to the radiators, there is also a mains-pressure, hot-water tank.

The system was running fine.
Then, from a downstairs radiator, I partially drained (I think maybe ~10li) from the system to add some inhibitor via the cistern such that it would get it right into the main system.
Upon completion, I refilled the cistern (and checked that visually) and then switched on the central heating again.

There is now no heating at all. I think the boiler must have fired-up very briefly and then perhaps shut-down as too hot. Then when I turned the room thermostat down fully the pump kept going (perhaps the boiler was calling for it?) but the pump didn't sound normal. I wondered if there was air in it.

The downstairs radiators are all still full of water. When I tried to bleed the upstairs radiators and the main bleed near the pump (on the feed into the hot water cylinder) I could get some air but then nothing more. I went around this bleeding cycle many times but never got water from any of them. I removed the central screw from the pump and got about 2ml of water but nothing more.

I've left the heating system off now because I'm not sure whether I can run the pump in air (if it is indeed in air).

I have had a quick look at the FAQs but cannot really find an answer. Sorry if I have missed something obvious.

I would be most grateful for any advice, please.
 
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CBW

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Check that there’s water in the expansion tank (cistern), If you’re getting some air, but no water it’s either not refilling or you may have a blockage in the cold feed.
 
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If you are not getting water from the air bleed valves on some upstairs radiators ,and the pump ,then you either have a severe air lock or the feed from the loft tank is blocked. I assume the tank has plenty of water in it.
 
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