Many thanks to the kind folk who have replied.

Theres more than one type of plastic plug
http://www.wiki.diyfaq.org.uk/index.php?title=Wallplug
some of which will be entirely unsuitable.

These are the type of plugs I've used (which I now read are low/medium
weight bearing) http://www.diydoctor.org.uk/project_...rd/58219_l.jpg

Always assuming that you can of course. *From the OP's description I'd thing
the picture has this sort of mirror fixing?http://www.screwfix.com/prods/14529/...s/Brackets/Mir...

Yes you are correct. These are exactly the type of thing affixed top
and bottom of the frame. As such you are correct in that finding the
joists and centralising the picture etc is impossible.

OP. *I assume the 4 mirror plates are there so two could be used landscape &
two portrait? *Also assuming the right plugs were used?

Are you suggesting I should have fixed two additional mirror hooks
portrait orientation on the upright sides of the picture? I have two
along the top and two along the bottom edges of the frame only.

Since the load is only downwards and not 'pull out' I'd say it would be
fine.

This was my assumption especially with 1 1/2 inch size 8 screws and it
is tight to the wall but I'm no expert hence the OP. Thanks to all