Do i need to see company doctor?

hi i have been signed off due to an injury at work (Back problem) and the ssp is being paid from ESA only not the company as i didnt earn enough, with ESA they require you to see one of their medical gp's, which is fair enough as they are paying me my wages basically, but my boss has asked when/or if i will be coming back to work? (how long is a piece of string) and that she wants me to see their company doctor. I would have thought that if they are not paying me SSP then why do i have to see their doctor? can i refuse on the grounds that i will be seeing an official medical practitioner? i feel like she is trying to call my bluff. thanks