Below is a letter I got after asking the library spends on proprietary software, I did not use the phrase "Open Source"
A pretty disappointing response that you got. Looking at the relevant page of the 2004/5 budget yields little (http://www.norfolk.gov.uk/council/departments/finance/Budget%20Book%202004_ 2005.pdf - page 20 of the pdf).
You could always try asking the Education and Cultural Services Scrutiny Review Panel (http://www.norfolk.gov.uk/council/committees/papers/schedules/education_cul tural.htm) to have a look into this for you. It might make a difference if it came from ALUG as an organisation. Both MJ and I know a bit about Scrutiny, though from rather different perspectives ;-).
Dave