I thought it was just the southern end of Soham that was missing, but no it turned out that around half the town wasn't done, including a whopping area of new housing on the northern edge. So it took me rather longer than I expected to complete Soham. Nevertheless it is done now - thanks to Donald for the larger chunk that was already complete. There's an interesting area to the east of the town called Qua Fen Common (strange name) and the similar East Fen Common, which seems to be a large tract of genuinely common land. It's well used for grazing ponies.
Then over the fen to Isleham, a village of some 2,500 people 5km east of Soham and one of the the last outposts of Cambridgeshire in the east. Isleham is known for its old priory, but I also discovered the Isleham Lime Kilns, a 19th century industrial relic in the middle of a housing estate.
By the time I got back to the bus stop for home (the last three miles across the fens into a fierce southerly wind), I was ready to stop after seven hours continuously on the bike.