Nigel the statements you said about a charger not needing a capacitor and the definition of the three basic requirements for a battery charger only apply to flooded lead acid batteries, that's a pretty important qualification which is why I mention it, if you try to charge anything other than a flooded lead acid battery using this better have a fire extinguisher nearby. Even sealed lead acids (Absorbed glass mat) will have a slightly different float charge which is critical to pack life.
Nigel, you assumed in post #10 that it was a lead acid charger, obviously it is, but it wasn't mentioned at any point previously in this thread, remember the average user that comes to these forums has very little to no experience with any of these things which is why it's important to fully define everything from the start, hopefully the original poster knows that this even if he did replace the diodes could never be used to charge anything other than a flooded lead acid battery, and that's if he's lucky =) Though when attached to a big capacitor, or even a battery it'd make one heck of a 12V power source.