My understanding is (but I'm open to correction...) is that both the rate AND the repayment are fixed.
If you take a 5 year fixed rate of 5% your repayments are set at x amount for 5 years. You pay an interest rate of 5% on the decreasing capital sum. So say you owe 95k after year one you are paying 5% interest on 95k (as opposed to 100k - the original mortgage amount)
I didn't explain that very well but I hope you can mae sense of what I was trying to say - also, I took those figures off the top of my head for the sake of the example I was giving.