Part of my work is teaching the people at banks who do the calculations for banks who offer fixed mortages.
They are typically very smart people, typically with a PhD in physics backed up with a masters in finance.
Part of the training my gang do is formal lectures in blackjack, roulette and conditional probability, one of the people we get in wrote the currently best selling book on poker.
www.amazon.com/Poker-Face-Wall-Street/dp/0471770574/sr=8-1/qid=1157449509/ref=pd_bbs_1/104-0906310-5234362?ie=UTF8&s=books
If these guys bet against you, then you have to expect to lose.
You may read in the papers that rates are expected to drift up.
This is sort of true, but the people at the banks know this as well, and the rate you will get reflects thet expectation, plus some extra cost. The extra cost comes from the fact that fixed mortgages are more risky for the bank, and this is for them a "cost" which they expect you to pay for.
So fixed mortgages are not a way of saving money, you wil on average lose money from one, and as RachH points out that could well be a big loss.
It is only worth having one if you are very close to the absolute maximum you can afford, and where even a relatively small movement could sink you financially.
In a two year period, you would not on average expect rates to go up much, and it's quite hard to see how the large set up and exit fees make it worth doing.
Over 5 years, pretty much anything can happen, we could be at 15% or 5, though not with equal probability.
However, the critical term is how much above current interest rates you are committing yourself to. So it may simply be worth taking a tracker and setting up a standing order to a rainy day account.
Also bear in mind that over a few years on average you can expect the value of your house to go up, and some small % of the debt to be reapid. That makes your mortgage in N years time less risky, and thus you can expect to pay slightly less when you change provider.
Disclaimer, I'm not an IFA, though I did build a good chunk of the system HM Treasury uses to manage the national debt.