In case anyone is interested, I've put a photo of the SD deinterlacing bug on the W5500 on Photobucket -
http://i709.photobucket.com/albums/ww99/spl237/sdi.jpg
It looks to me as if alternate blocks of horizontal lines are in the wrong order. This only happens with SD interlaced content over HDMI - SD progressive, 720p, 1080p and 1080i are all fine over HDMI, and SD interlaced is fine over component and SCART.
I think this has to be a software bug (ie common to all of them) rather than a fault with my particular set, unless anyone knows otherwise.