I came across this some weeks ago and finally got around to making one from one of the Cisco PSU I have.
http://www.nightfallcrew.com/?s=amstrad+cisco++power+supply (http://www.nightfallcrew.com/?s=amstrad+cisco++power+supply)
I did a slight mod that might interest people which made it much neater for me.
I connected a panel mounted socket for the 12 volts like used on the monitors.
Much neater if you only have a 464, no stray unused cable lying around.