I have several of these. The best rate for monthly fixed rate at the moment is around 3.75% and bearing in mind that interest rates have been lower than this a few months ago and are now rising slowly, I resisted the temptation to take a longer term than 1 year. You can get a higher rate for longer terms but I would suggest that by the end of 2 years, you will definitely be losing out. Best to lock it in for a year then look around for a better rate when it matures....
Good luck