Hi Aidan.
There's a transparancy slider thing in Google Earth. You can drag the slider from 100% clear to 100% opaque.
So you have present day Birmingham on your screen.
Click 'add overlay image'. Choose old map from your PC.
The old map overlay apears but the clever thing is, you can drag around, stretch, rotate or compress the image to fit the present day land marks. Use the transparency slider to help.
If you loose the 'image drag frame box' (not sure what it's called) just
click the overlay image title. Click properties to edit & improve the position of overlay.
gary.