I would start now sorted, I recently had a letter from my lender saying my current deal would end in some month's time but I needn't wait until then to go and talk to them about what they could offer me.
Might be an idea to have a look around at some of the deals you might get if you were to switch, then when you go to see them they will know that you've been looking around and are more likely to offer you something competitive to keep your custom.
I know my first lender didn't bother even trying to offer me incentive to stay with them, but the ones I'm with now said they were very surprised at that when I joined them, and that they tried to keep people with them.
Good luck, I'm sure you'll be okay, and if you do end up switching it's really no hassle whatsoever. All my new lenders did at the time was send a surveyor round to check the valuation (which might not be applicable now as at that time I had little or no equity as house prices had been stagnant for a couple of years after I bought it), and I didn't have to pay anything.
*It is not necessary to understand things in order to argue about them. -- Pierre De Beaumarchais