Here is some additional feedback. Seems like one can reproduce the problem by comparing the LS log() results with Excel's log() function:
"I have now checked everything and the bizarre result seems to be that LS gives log(67) as 4.2 whereas excel and other internet sources say it is 1.8!"

Seems using ln and not log ...

Not really a bug: it's default behaviour for php log AND for javascript log.

And:
"If not specified, the assumed base of the logarithm is either 10 or e, depending on context." ( http://en.wiktionary.org/wiki/log).

Question :
Don't change log(x), but put optionnal setting ?

• Add ln + lg

Atually, you can do
{log(VALUE)/log(10)}

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=11576

Actually:

• use {log(VALUE)/log(10)}
  Next update:
  {log(VALUE,10)}

Fix committed to 2.1 branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=11610

Documentaion updated too.
http://docs.limesurvey.org/Expression+Manager#Access_to_Functions

The logarithm of number to base, if given, or the natural logarithm.

New build 130213 released.