Me wrote:
Help! I'm in deep trouble - have got our lad a manual focus 500mm Tokina
mirror lens to go with his new Samsung gx1-L digital slr.

A clone of the Pentax *ist-DL, a nice entry-level Pentax DSLR that is
made in partnership with Samsung.
I own a Pentax *ist-DS which is almost identical to your son's DSLR,
except that it has a glass pentaprism and more AF sensors.

BUT it's Christmas day and we've now got a teenager wandering around
claiming that it's not a good idea for a present as it doesn't work!!!
He seems to think that its a problem with the metal connecting
flange/plate on the mirror lens contacting the metal contact buttons
on the camera body that is causing the camera to go apoplectic...
any ideas please before he brains me with the mirror lens????

Those contacts tell the camera the aperture range (how far to move the
aperture lever to stop the lens down to the required or selected
aperture) of Pentax KA and KAF lenses.
Your lens has a fixed aperture (f1:8, if my guess is correct from your
description) and no electrical contacts, so the camera can't tell what
the aperture range is and throws up an error and won't take a picture.
The menu option to enable the aperture ring needs to be set (even if
there isn't an aperture ring on this lens).
Your lens is also (probably, I don't know) manual focus, so the focus
mode must be set to manual.
Otherwise, the camera will throw up an error and the shutter won't fire
unless part of the scene that is under an autofocus sensor is actualy in
focus.

He almost smiled just then as he has put a piece of plastic bag
between the camera lens and body/contacts and it now takes pix - well sort
of he says ...
but they look alright to me..
maybe its time to bodge a little????

There are two things that should be done:
1) set the focus mode to manual.
2) go into the setup by pressing the menu button and activating the "use
aperture ring" option.
If you don't know how to do this, I suggest that you read the manual
that came with the camera.

If I have cross-posted to the wrong groups please feel free to advise which
ones to try....

Most of the newsgroups that you posted to are not carried by most
internet providers, mine only carries rec.photo.equipment.misc &
uk.rec.photo.misc
You should also look at alt.photography (which I can get) or
rec.photo.digital (which I can't).
An alternative is to look at the forums on some of the digital camera
review sites.
The one I visit the most is http://dpreview.com.
They have a Pentax DSLR forum where you could probably pick the brains
of the members for your DSLR questions.