The lease should start at a mutually agreed time. He cannot force this to be backdated. You would have probably had security of tenure and be entitled to a new lease at a market rent on terms broadly in line with your previous lease. If you cannot agree terms then you can serve a S26 notice requiring a new lease and then apply to the court if required.
The opinions I give are simply my opinions and interpretations of what I have learnt, in numerous years as a property professional, I would not rely upon them without consulting with a paid advisor and providing them with all the salient facts BSc (Hons)