P11Ds are used to report benefits provided and expense payments made to you that have not been taxed through the Payroll. It HMRC of any 'Benefits in Kind' received by a HCP for such things as Accommodation (Box D) and Travel Costs (Box N).
The figure reported is made up of the total amount paid out by ID Medical minus anything that has been made good (recovered through payroll) from yourself. Please make sure you keep your P11D safe for future reference.