If you have downloaded some Birdseye 1:25k Mapping to your GPS Device, 

To be able to view this mapping on your device you need to first go into Setup - Map - Configure Maps and Disable your 1:50k GB Discoverer Maps.

This is because the Garmin GPS will always read the Maps loaded on the SD card first, when you want to view your 1:50k maps again remember to repeat the process above but this time select enable your 1:50k GB Discoverer Maps.