Never fix your mortgage, it's almost never worth it. Take the best discounted rate you can get and you're likely to be better off. Fixed mortgages are a bet, you against the mortgage company, on the levels of future interest rates; but like most gambling, the house always wins because the game is rigged in their favour. You're essentially paying a premium for them to take the risk of interest rate changes instead of you.
So unless an interest rate change actually threatens your ability to pay, take the lower rate, put it in savings and if rates go up you can use the extra money you've saved to offset the increase and end up, 9 times out of 10, up on the deal.