Under the Data Protection Act, the data controller is the person or organisation that will decide the purpose and the manner in which any personal data will be processed – they have overall control of the data they collect, and decide how and why it will be processed.
A GP Practice is a data controller for the patient information it collects, and should already have data processing arrangements with third parties (e.g. IT systems providers) to ensure they do not use or access data unlawfully; the data controllers will have ultimate responsibility for the Practices’ compliance with the DPA.