The Fit For Work service has now been rolled out nationally and guidance has been updated for employers to reflect that from 8th September 2015 employers can accept return to work plans issued through the Fit For Work service as evidence of sickness absence in exactly the same way as a GP fit note.
You will recall that under this new system the GP may refer the employee to the service, effectively washing their hands of the employee, passing them on to this new service which is aimed at focussing on getting that person back to work. If, upon an assessment (done over the telephone), the occupational health professional deems that the employee is indeed not fit to return to work, then they are likely to issue a note which the employee can then give to their employer to evidence this fact. It would not, therefore, be appropriate for the employer to refuse to accept that and, for example, require the employee to get a separate note from their GP. In any event, GPs are likely to refuse to even see the person as they will have discharged them into the hands of this service.
You may be interested in a booklet that the Government has published which is at https://www.gov.uk/government/publications/fit-for-work-guidance-for-employers
You may need to update your sickness absence policies to tell staff about the advice that is available from the Fit For Work website or telephone line, to explain that they may be contacted by Fit For Work if they’ve been referred by their GP, that you as employer may consider referring them to the service and what they should do if they receive a return to work plan from Fit For Work and how they should endeavour to comply with any recommendations made.