I can see how the packing piece of wood works especially on an unplastered wall where a temporary piece of wood can be attached to act as a datum whilst installing. However installing the boxes individually with just a packer in between still has the potential for some vertical misalignment.

When putting three double sockets in a line behind our TV unit I used two steel strips pop rivetted to the back of the boxes which if I remember correctly were spaced 15mm apart, worked a treat.

I am just looking for a simple but accurate means of aligning a number of sockets and switches as the walls will be tiled and nothing shows up dodgy or unlevel alignment than tiles! I suppose as someone has pointed out there can be some variation in metal faceplates especially the low profile type. We actually intend to use profiled accessories so it will probably mean checking dimensions and suck it and see. If I have to try out a few back boxes so be it they are not the most expensive items in this rewire.

Richard