I'm in East Anglia which is the bit that sticks out into the North Sea to the north east of London although I'm originally from Hertfordshire just to the north of the smoke. Plenty of change ringing around here
Did you click on the picture link to load the video? I don't have it in front of me but at a guess I would say the compressor is between 8-10 inches long by 3-4 in diameter. There are no pistons it's basically a three section unit. First is the motor above which is the next section - a sealed concentric chamber into which the motor shaft passes. Mounted on the motor shaft is a metal cylinder with slots into which three sliding steel vanes are held, the motor shaft comes up somewhat off centre so that the vanes slide in and out as they rotate. There is a filtered air inlet on one side of the chamber and air is drawn through this and then blown into another chamber above by the vanes under pressure. With me so far?
The top chamber contains a sealed reduction gearbox driven by the motor shaft below. A shaft that protrudes from the top of the gearbox then turns a slotted disc that is held under spring tension against the top plate of the chamber into which the two horn hose outlets are fitted. The slots in the disc correspond with the air outlets therefore forming an air valve so that as it rotates it alternately shuts the air supply to each horn on and off.
In principle it?s a lot simpler than my description