It seems from a number of posts that it is fairly common for Blaupunkt head units to come on by themselves - draining the car battery. Here is a cheap and dirty solution:

Buy ( Maplins / eBay / Halfords ) http://www.ebay.co.uk/itm/Draper-5m...ter-Adapting-/400434310304?pt=UK_BOI_Electric £6.90

Pull radio out of dash ( need extraction tool or some stiff wire ).

Replace the main radio fuse with test lead.

Connect a switch to other end of lead.

Route behind dash - this is the hard part.

Now the new switch will completely switch off radio.