Got a rejection from the agent in London this morning. He was fairly positive about it -- described the manuscript as "well and intelligently written", and said he had no editorial comments to make. The reason he gave for not taking it on was that it just didn't quite excite him enough. That's sort of thing is purely a matter of taste, and it's impossible to legislate for -- that's one of the main reasons finding an agent is so hit and miss. He didn't seem to have any objections to the manuscript being a partial sample rather than a finished novel, which was a relief. He also encouraged me to send other stuff to him if I develop any, and said he'd put me on his list of candidates for 'work-for-hire' contracts -- he also acts as a licensing agent for computer game properties, and when he sells computer-game tie in novels to a publisher, they often need an author for the project too. So I guess he really can't have objected to the quality of my work that much... Even so, it's still a bit of a blow.
The next targets in my sights are an agent in New York who has agreed to take a look; and a publisher in London who I've sent stuff to before.