Well I tried the Stant 10229 radiator cap today & unfortunately it wasn't a success. I attempted to carry out the cooling system bleeding procedure but as soon as any kind of pressure was built up the cap started to leak. I did notice that the cap wasn't completely tight on it's seat when tightened fully, you could still push on one side of the cap & it would vent coolant. The outer seal isn't hard up against the underside of the cap so allows some flexibility in the seal, too much it seems, whereas the original's outer seal was actually hard up against the underside of the cap, allowing no flexing. I tried the original cap again & you can't move it at all once done up. So it's either a faulty cap or it's just not suitable. I'll try Paul's suggestion of the Gates RC135 next.
2nd instalment on the alternative radiator cap ...and a self kicking moment
I tried the Gates RC135 cap today & that leaked too as soon as any pressure was built up. I was racking my brain as to what the problem could be, then it suddenly hit me that the pipe from the radiator filler neck to the expansion tank might be blocked ...which it was. D'oh! When pressure builds it's vented into the expansion tank, but if it's got nowhere to go it'll vent from the cap instead of course. The pipe was blocked solid, I managed to blow the obstruction clear & flushed it through a couple of times. I used the Stant 10229 cap that I'd bought previously as that's rated at 13 psi (0.9 bar), the same as the original Kawasaki one. I carried out the coolant system bleeding procedure & all was good this time, no leaks from the cap & it vented the air into the expansion tank as it should, as well as drawing coolant back into the radiator as it all cooled down. So the Stant 10229 is a good alternative radiator for our bikes after all. And fellas, check your expansion tank hose for blockages! Lol.
Btw, just out of interest, the Stant & Gates caps appear to be identical in construction, even down to the "Assembled in Mexico" & "S" logo stampings, so I wouldn't be surprised if they're made on the same production line.