No they can't.
To be fair to your employer, if you are able to give them a rough idea when you will be fit for work you should do so. especially if it is a small firm.
A friend of mine is off work with a broken arm. She went in to hand her Doctor's note in and to tell them she'd be off for 6 weeks. The boss saw her, handed her a pile of documents and said "Type these for me while you're here"
If an employer believes that an employee is taking a longer time to return than would normally be expected they can contact Her Majesty's Revenune & Customs to request help from their Medical Services.
The employee has to give consent for them to contact his/her doctor and ask for a report and they may also ask the employee to be examined by the Medical Services.
The employer may cease paying SSP pending a reply from HMRC who will say if the employee is or is not capable of work.