i've just went through this exercise too – we still only have < 10% equity having bought at the peak of the market 🙁
Went from a fixed rate of 6.2% to the current variable at about 3.5%, and are overpaying the difference from what we were paying on the fixed rate. I've decided to stay with the variable for the forseeable future, because we have no equity the fixed rate deals available are pish and the fees are astronomical (£1000!) which would be added to the mortgage. So it worked out about 20 quid a month more to fix and we're not getting the benefit of overpaying with that.
I agree that if interest rates do rise, it will not be by more that 1/4 or 1/2 a percent, as a jump greater than that would have a big impact.
Basically the best thing to do is work out how much you can afford just now, overpay if possible to get the capital value of the loan down, and watch for the interest rate rising to the point where it would start to cost you more, then fix. Of course the downside of that is that the banks will be one step ahead so will likely make sure the best deals are long gone before it starts to bite…
tough call.