At the end of March 2008, the latest date for which figures are available, there were 155 foreign national prisoners (101 male and 54 female) detained in Peterborough prison.
This information is available at the following website where it is updated quarterly.
http://www.justice.gov.uk/publications/populationincustody.htm
http://www.justice.gov.uk/docs/population-in-custody-mar08.pdf
These figures have been drawn from administrative IT systems, which, as with any large scale recording system, are subject to possible errors with data entry and processing.