The diffuse channel (1st channel) can overlap and do whatever you want but the lightmap UVs (2nd channel) need to be unique. They should not have an effect on eachother :D
A rookie question; but could you tell me what the map types are on the shield you made? I recgonise Normals, Diffuse & Specular but unfamiliar with the other - is it a gloss map?
Yep, one set of good UVs for the Diffuse and normals, etc. And then I have a second UV set that is non overlapping. Is it something to do with normals overlapping in UDK, maybe?