Here is the current official Liberal Democrat party logo, with and without strapline:
The set of pdf versions of the Liberal Democrat logo also include the Scottish and Welsh variations.
Or, for serious graphic design uses, you can get the EPS versions of the Lib Dem logo here. As well as the Scottish and Welsh variations, this set also includes the version to customise by adding local party names etc.
P.S. You may also find the Liberal Democrat style guide useful.