Without doing an in depth fact find and knowing your full circumstances it is very hard to say what is the right and wrong answer.
Personally, as the interest rates are so low art present, but also in the knowledge that they are creeping up due to the turmoil in Europe and knowing that 5 year fixed rates are about as low as I have ever seen them I would be very tempted to grab a 5 year product and finish the mortgage with it.
Do also remember that as your mortgage is most likely now down to the £40k - £50k mark is starts to be come more cost effective to take a fee free mortgage product that will attracty a higher interest rate than a fee paying product with a lower interest rate, Adding a £999 fee onto you mortgage is adding 2% to your balance and this may well take 3 - 4 months just to pay it off where by if you paid an extra £5 per month for a higher rate but just paid directly off your existing balance then the balance it is only a small increase in rate to cover and not an increase in balance and possibly term
Nationwide BS are offering a 3.79% 5 year fixed rate with only a £99 booking fee and free legals and valuation. There are other prioducts that are even cheaper than this but often carry a fee or may be difficult to communicate with or get to a branch of their bank.
I would go and see a mortgage broker and discuss further