Q: Bluetooth mouse and keyboard in Target Screen Mode

I have recently started using my iMAC 27" as a Screen for my Macbook Pro (Target Screen mode using thunderbolt) but am having a complete nightmare with magic mouse and Apple bluetooth keyboard.  Whenever I start a fresh session neither bluetooth device works. Both are found but will not pair. If I go through restarts I get them working again but the connection only lasts as long as I am actively working. Let the machines go to sleep or even screensaver and the bluetooth is lost and does not restore in the normal way.  Any suggestions?