Your going to have to start paying that capital back sometime - and now is probably the best time to start with these low interest rates. Although - there is also the option to pay amounts into a 7% regular saving account instead for a while - and then use the lump sum built up to pay off a lump sum on your mortgage later . But the temptation of spending the savings might be a problem?
A savings account where you have to pay in a regular certain amount in each month for a certain amount of time. They are known as regular savers - and many offer rates of 7% or more for example 8% at Anglo Irish . These rates of course may not be around for much longer...