Colin/John,
Thanks for your responses..
I was trying to do without the rail - sort of magic levitation stuff :-)

Normally when I hang pictures that don't need to be changed
occasionally, I would hide the hanging apparatus behind the
picture. That's the look I want to create, but I also want the
flexibility to change.

Hence my thoughts of having a small button on the wall that
is removed to reveal a threaded hole. I can then attach some
hanging mechanism that will end up behind the new picture.

Wall has got to be plastered in the near future so I can hack
about at will. I was considering burrying a small stainless
steel bar into the length of the wall with tapped holes in it a
regular intervals. This would be screwed to the breeze block
then covered with plaster but with the tapped holes clear.
Then there would just be a row of small buttons showing
along the wall. Might be an interesting feature in itself :-)
I wonder if I could manufacture some square headed buttons...

I wonder if the plaster would crack as the temp went up and down?

As you can see I'm going off into all kinds of weird fantasies :-)
I suppose a rail would be easier...

Hmmm...
Roy