Due to the high memory usage issues and no apparent benefit, I
uninstalled 64-bit Jitsi and installed 32-bit Jitsi yesterday and
guess what, after all this time thinking it was incompatible it now
recognises my PS3 Eye webcam. 
So I guess the driver is only 32-bit and so 64-bit Jitsi couldn't see
it or something like that and perhaps this applies to some other
webcams that people are finding are not working in Jitsi x64.
Regards
Derek