The rate on your offset is 5.15%, the rate on a below 80% LTV tracker is 4.75%. This would probably account for the lower term. However if the only use you have for an offset mtg is to pay extra monthly payments then it is not of benefit to you, you could do that with a tracker. The offset mtg can have up to 5 accounts linked to it on which the balances are offset against the balance of the mtg before calculation of interest. If that sort of arrangements suits someone then it can work well, however if you just want to pay higher monthly payments you dont need an offset.