snip
I tried a level from B&Q. Width of line was about 4mm. I had hoped for a
razor sharp accurate line. No such luck. A distant memory from the
optics section of my engineering degree suggests it's to do with the
"mode" of operation of the laser. Maybe a high quality laser gives a
finer line.


More likely to do with (deliberate lack of) collimation. The optics are
probably set up so that the beam diverges - otherwise, the laser might
take your eye out!