Update on my fuel overflow problem. I lapped all the valves, which took absolutely ages and then reassembled the engine and checked and re-adjusted the valve clearances. I started it up this afternoon and it's still bloody leaking.
In desperation I disconnected the fuel pump and started it up again, and it ran absolutely fine until the carbs emptied. So it doesn't seem to be a back-pressure problem, but rather a problem on the inlet side of the carbs. I've already replaced all the float valves and seals, so I'm at a bit of a loss as to what the problem is.
Interestingly, when I dismantled the bike last autumn, I discovered that the wires to the fuel pump were connected in the classic bodger's "twist them together and wrap tape around it" method. This leads me to believe that the pump isn't exactly the correct one for the bike. It may just be that it's been replaced with a 98-01 pump at some point, or it may be a pump from a different model entirely. Does anyone know if the connectors are different between the different years? I know this is the case for some parts of the wiring loom, as my ebay engine had different connectors on the oil pressure sensor and the alternator.
My first worry was that I might have reconnected the pump backwards, but I'm pretty sure that it wouldn't run at all if I'd done that. The pump seems to be operating correctly, ticks a few times when you turn it on, then stops.
Next step is to try plumbing the fuel in direct from the tank to the carbs and see if it'll run properly then.