You should ask the relevant probation service for a 'subject access request form'. (It's not strictly necessary to use their form; you can simply put your request in a letter. However using the form ensures that you'll do everything required). You should note that the Data Protection Act (which is the relevant legislation here, and not the Freedom Of Information Act) permits a charge of up to £10 for providing the information and, in practice, that's what is always charged.
Specimen form here:
http://www.glosprobation.org.uk/Resources/Gloucestershire%20Probation/DPA%20Subject%20Access%20Form%20Data%20Protection%20Act%201998%20v2.pdf
Prior to taking the formal route though it might be worth simply asking your probation officer what he/she will let you see. You might save yourself a tenner!