Paddy. The chance of the problem being caused by an air lock is practically nil as the system is pressurised.
From the description it sounds like the quantity of water (pressure) flowing from the tap is insufficient to maintain sufficient gas supply to produce hot water.
Check to see if you have a stopcock on the supply to the tap, if you have, check that it is fully turned on.
Most taps in Spain have a metal filter, which is screwed onto the tap where the water comes out. That often becomes blocked with calcium particles (like the fur in a kettle). A build up will reduce the water flow. If you have a filter, remove it and clean, or you can buy, very cheaply, a replacement from a ferretería (hardware shop).
That you drained the heater also might mean some deposits in it were washed into the hot water supply pipes. Which would also add to blocking the filter.
If you have never cleaned the filters on the other taps I would do that too to avoid future problems
I just found this web page. It explains better what I have said:-
http://commonsensehome.com/how-to-remove-hard-water-buildup-from-faucets-and-showerheads/
This message was last edited by johnzx on 01/08/2015.
This message was last edited by johnzx on 01/08/2015.