Thank you all for your help and advice.
The plumber said he'd call back but didn't for a whole week(!). In the meantime I was planning to get hose and other equipment. And then all of a sudden voila! The 2 rads have started working again today for no apparent reason! The airlock must have finally moved out. I am pleased, but concerned about it happening again in the winter. Hopefully I won't need to have the system drained again so it might not happen....
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