If anyone is thinking of installing the disk interface components into the Amstrad 464plus, the SED9420 is currently on ebay, I'm not sure if it is exactly the right variant of the chip, so you should check it first.
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=280686346210&ssPageName=STRK:MEWAX:VRI (http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=280686346210&ssPageName=STRK:MEWAX:VRI)
It is the correct part.
Bryce.