It's pretty good in the current climate, but is there a fee attached?
Sometimes it's worth paying a fee to get a lower rate, but that depends on the mortgage amount and length of fix. It probably wouldn't be worth paying a £1k fee for a 2 year fix on a £100k mortgage, but it might be for a 5 year fix on a £300k mortgage. There are calculators on the internet that help you work this sort of thing out. Also put your circumstances into Moneysupermarket to see what rates you qualify, or talk to a broker.
Also, are you certain of your LTV, because at 79%, it only takes a small reduction in what your lender thinks your house is worth for the LTV to increase past 80%, at which point, you might not qualify for the rate you are looking at.
We're also on a historical low margin tracker, so are paying 0.48% (carer if you're 0.35% above, you're actually paying 0.45% as base rate went down to 0.1% remember
) but we're not overpaying as can still just about get more interest in savings - managed to get £2.5k at 5% from Nationwide before they pulled it, which sounds like an astonishing rate now - that £2.5k is earning more interest than what about 70% of our mortgage is costing us in interest. Plus the rest is more than covered by premium bond prizes.