I would like to request information on the number of MPs who currently take on unpaid interns, the amount of expenses they are paid, and the type of work that they do.
In response to the first part of your request, I can confirm that there are currently 261 interns/volunteers registered by MPs. Details of reimbursed expense claims made to specific MPs for the use of interns are published on the IPSA website. The information you have requested is therefore exempt from disclosure under section 21 of the FOIA (information accessible to applicant by other means) because it can be found on the IPSA website.
Information regarding the type of work undertaken by interns will be held by the MPs themselves. It is possible that the FOI team at the House of Commons may be of some assistance. The attached link to the Freedom of Information section on the House of Commons website will assist you with your enquiries: http://www.parliament.uk/site-information/foi/foi-general/commons-foi/.