I've contributed an alternative method for implementing a side switch. It worked like a charm for me and surprised it hasn't been covered before. Hope it helps someone!
http://www.cpcwiki.eu/index.php/Side_Select_Switch_-_Alternative_Method
Cant wait for some pics, I'm lost as to where to stick the microswitch!
The CPC disc ready signal can be switched by hijacking the HD switch on the fdd board, bit of track chopping required, yep it's all good.