I'd also say it is just an airlock.
Nothing for you to do regarding the system. All you can do is to try to clear it. Togo's methods are tried and tested. It's a bit of a game though. Assuming the kitchen has a mixer tap, I usually open the hot tap, place a large Builder's gob over the spout.......... and blow. There shouldn't be any need to monitor "progress". Keep blowing, and eventually, hot water should start to stream from the hot tap.
It's a bit hit and miss, I'm afraid, but you should get there eventually. A farmhouse (it's often old farmhouses) that's been "renovated", but incorporating an old-type gravity system, can throw up all kinds of problems. It's often down to poor pipework layout. Critical with gravity systems.
The main way that air can get in, is if the roof cistern (tank) runs dry. That is, running a lot of hot water (maybe a bath), when the supply to re-fill the roof tank is unable to keep up. Possibly from an undersized tank, or low mains pressure. Just a possibility.
Re-jigging the pipework layout properly will usually sort this out, but that's beyond your control I'm afraid.