Cheers for that. The lines run parallel with the back of the baseboard. They
appear to be quite thick. Sliding the red sliders has no effect.

FireBall

If the lans is cracked does anyone know where I can get a replacement?

Sorry, for the blank post, late at night here.  The 601 uses a reflex mirror
to bend the light 90 degrees down to the lens.  The are two condensing lens,
the top one can be swapped out for 50mm lenses or 80mm lenses.

If it's got the B&W condensor head on it then it has a mirror in it.  With
the B&W head it should also have two condensors, and if you want to do 35mm,
a third one.  Alternatively, it may have the 35mm combo and you'd need the
other one for MF.

So, is it a B&W model? If so, remove the neg carrier and examine.  Check the
action of moving the red tabs to slide the masks around.  Unscrew the lens
and look threw it.  Next, remove the filter drawer (red thing at the front
of the head), open the top up by rotating the tab at the back of the top
plate, lift the lid and turn it over... There's your mirror, angled at 45
degree to bounce the light from the globe down the condensors.  With the
power cord unplugged, remove the globe, undo the locking clamp screw, remove
the clamp and lift out the condensors.  Examine them to make sure their in
good condition and see if they have some writing on the sides of the metal
frames.  They may be labeled Sivocon 80 or Sivocon 50.  You use 2 80's for
MF and one 80 and one 50 for 35mm.  The '50 has a lot more curvature, so if
you have two the same, they almost certainly be two '80s.  (do a search of
the photo.net B&W Printing forum for discussion about what happens using 2
80's and a 50mm lens for 35mm).  With the lens, neg carrier, condensors and
mirror out, you should be able to see any defect causing black lines.

So along the lines of what Nicholas said, without determining exactly what's
causing it, don't buy it.

Cheers, Nige

To quote the key point from my original post:

"I have spotted a Durst M601 enlarger at a local auction house" ie not a
shop.

Congratulations on picking up a fine enlarger at a great price (I have two
'601's).  Great machines and the good news--inserts, AN glass, and
lensboards interchange with newer models--just don't break a condenser!

congrats!  Did someone actually bid against you?

Nige.