Thank you for the comments chaps.

Rabbitz, here's a couple of pics I haven't used in this thread as they are poor quality. With one of the walls I'll overlap the brick sheets on both sides by the thickness of the foamed PVC sheets I use for the main structure (in this case 3mm).

With the wall I'm going to connect to it I'll just stick the brick sheets flush with the edges as no overlap is needed. When I fit the two sections together this wall section will fit into the overlap. There will be a tiny gap left but some filler sorts that out.

Here's a very crude diagram which probably explains my babbling more clearly:

Clear as mud?