In reply to Removed UserGRUMPY MONKEY:
Even if sickness is not related to an underlying medical condition you should not be subject to a Disciplinary procedure and local authorities should know better. Check your NJC book and look at the procedure for Capability & Diciplinary.
Disciplinary = Won't Do
Sickness = Can't do
That said if the employer chooses not to believe or acceept a sick note they are legally entitled to not accept it. But what normally happens is they'll use an occupational health service to get a second opinion. But from my general experience is employers very,very rarely refuse to accept a clear statement from a GP on a fit note.
If this was raised your line management I'd have an informal word with your HR dept as thy'll know better (or should do) - if it was raised by your HR department I'd consider raising a formal grievance with your employer and this should prevent his getting out of hand. Your LA weill have a written grievance procedure which will be in the JNC book.
If push comes to shove you will win because the employer can produce no evidence to support their argument.
(I'm an ex - HR Manager and employment law advisor and member of the CIPD)