Bike Shop
In the late afternoon I took my bike to a local bike shop. The guy there didn't speak much English, but was super-helpful!
He bent the big chain-ring of my bike back towards being straight (got bent on the flights to China/Japan), adjusted the gears so I can access them all without holding a lever in and even helped me with re-fixing one of my umbrella poles so I should be able to use my DIY rucksack mount again (it been broken several times now, but definitely more comfortable than wearing the rucksack while cycling, so hope to keep it going a bit longer!).
I also bought some chain degreaser/lube and spare gear and brake cables. All for about £20!
I had been under the impression we needed to register our bikes at a police station or bike shop in Japan, but he didn't seem to think that was necessary.