Not unpowered, but one of the Currie Izip models has exactly the same battery in frame tube as the Sparta, and also a rear hub motor, though not the same motor:travelling in the opposite direction to me on Norwich Road (Mid Suffolk towards Ipswich) - a chap in this 20s/30s on what appeared to be a Sparta Ion (do any non powered push bikes have the same distinctive curved frame?)
They do the gents version as well, and that makes me more sure it was this one:think it was like that but had a gents frame - didn't see or hear the chain driven motor - incidentally I've noticed the level of traffic noise is such with todays more modern looking designs many onlookers don't even realise you are riding an e-bike!