How long to fix?

oligopoly

I think AnotherJoe made a pertinent point. The younger you are and lower down the ladder you are (so to speak), often the greater probability that your circumstances will change. I know plenty of people that moved from their first property within 2-3 years (myself included). Marriage/kids/jobs etc are all factors which can change your life and objectives completely.

Be prepared that whilst you can usually port your mortgage across should you wish to move up (or across) the ladder, high exit fees may force you to stick with your current lender. If they won't lend you enough then you may find yourself trapped.

Personally I have always preferred the option of a short (2-3 year) fixed deal - it gives you greater flexibility and no-one know what the future holds...