Hi Sarah,
Without fully understanding your circumstances it is difficult to give you advice, but I can give you some general things to consider in making a decision.
The main question is are you willing to pay a premium for the certainty of a fixed rate, or would you prefer a cheaper rate that has an element of risk.
I would suggest you look at the tracker option, see what it will cost you now, and then do some calculations assuming a 1%, 2% & 3% rise in interest rates. If payments at this level would be uncomfortable, then perhaps the fixed rate is a better option, however if you would have no problems what so ever with the rate increases and you can afford the risk then it may be worth while considering the tracker. Ultimately the decision is yours, and as I said above, without conducting a full factfind I can't really advise.
On a side not, is this what your current lender is offering you? If so, havbe you shopped around? As there may be lower rates elsewhere.
I would suggest contacting an independent
mortgage broker to see what alternatives are available before you make a decision.