Once your 6 months expires, you automatically move onto a "Statutory Periodic Tenancy". This is exactly the same as your current agreement except you must give 1 months notice and your landlord must give 2 months notice (both ending on the same day of the month as the original 6-months ended).
If you are sure you want to be tied down for a further 6 or 12 months, than ask the landlord for a new fixed term. If you want flaxibility, leave things as they are, but bear in mind that this option gives the landlord flexibility too.