Martyn - currently we don't support publishers adding custom layers (like local authority boundaries) to our offline vector maps unaided. We can do it for you, but there would be a charge (mail me for details), and we can't have everybody piling their special-purpose layers into our general-purpose vector maps (althought UK local authority boundaries are pretty general). If you don't need zooming, you can use a static image map that you insert as an image.

In version 2.6 and beyond, we are adding more options/sources for the zoomable tiling map backgrounds (with some layers) from open source mapping services. Some of these may have or soon have the boundaries layer you need, or could help you add them in the spirt of open source data. Although zooming with tiling services does not work offline as vector maps do, in practise, if you zoom into all the most relevant locations, the mapping tiles will be saved on your local machine and the zooming will work offline to an extent.

Thanks Tom, sounds as though 2.6 will do it for me.