Answer:

The number of first-degree language undergraduates in UK higher education decreased at the end of the 1990's. the number of First-Degree students in languages in the year 2008/09 has shown a slight pick up (2%) although compared to 2002-03, it is still down by 3%.

CILT analysis of HESA data (Student headcounts) - time series 2002/03 to 2008/09:

Time series on first-degree, other undergraduate and postgraduate students in languages. Contents include student profiles, subject combinations, language and regional anslysis.

All UK and overseas students enrolled across all years of study at HE institutions in the UK. This is headcount data recording each language student as 1 whether they are studying single, joint, major, minor or triple honours, hence measuring how many students are studying each language.

Click here to read the report (2,202kb).

DIUS analysis of HESA data (Full person equivalents):
HE language student enrolments in the UK, 2002-3 to 2007-8, by language and qualification type (Word doc, 192kb) 

All UK and overseas students enrolled across all years of study at HE institutions in the UK. This analysis uses the full person equivalent (FPE) count where single honours students = 1, joint honours = 0.5, major honours = 0.667, minor honours = 0.333, triple honours = 0.333

Data source:

HE registrations data is collected by the Higher Education Statistics Agency (HESA) from their Student Records of individual enrolments at HE institutions. The accuracy of the data depends on how individual institutions code and report their figures to HESA.

Data notes:
In 2002/3 HESA introduced a new subject classification called the Joint Academic Coding System (JACS), hence data post 2002/3 is not comparable to previous years.

For another different source of HE data, please see this Higher Education Statistics article.