You need to be very careful
The early years of any loan or mortgage comprises of a higher proportion of interest. This means that if you re-mortgage at a slightly lower rate, you could actually be worse off in the long term. The new deal will start fresh, so again you are paying a lot more internet than actually digging away at the loan.
I think the guardian has a loan calculator which breaks everything down by month. You can see the capital reducing faster in the later years.
That said, we were in a similar situation a few months ago.
We have a santander mortgage which was fixed for 5 years. Personally I like fixed rate deals as I know how much it is every month. They are not for everyone, and they come with an ERC (early repayment charge). This means that if you are not out of the fixed rate deal, you will be stung with a % to end the deal early. This again needs to be taking into account when thinking about looking at a new deal.
Something else to also think about is if you can actually get accepted for a new mortgage. Banks have become stricter when lending money. Just because you got £100k a few years ago, doesn't mean you can now. Worth a phone call.
Lucky for us, Santander had already agreed a better deal for us. When our fixed rate finished, I phoned up to ask the question about getting a better rate. They looked on the computer and noticed it has automatically accepted us for the sale deal, but over 2% less internet. That for us meant a £250 saving a month
It's another 5 year fixed which I like. Love the house, kids are settled, both our jobs are secure (public sector education), so it was a no brainer.
I did put the figured through the guardians calculator, and even starting a fresh deal we are saving in the long run as well.