I picked up this Electronic Pendulum Kit (TJ-56-678) with a bunch of other random kits from the far east and, as usual, it was totally forgotten about in my box of shame!
I built this one on camera in case it helps anybody else, because the instructions with these kits aren’t always great. The really fascinating bit is the hall sensor, which sits between the coil and the magnet on little bits of wire. It takes some fiddling to get it all lined up, but once it is together it tick-tocks away all by itself. Job done!