The 15% boost is an interest-free equity loan which is frozen for 5 years.
An equity loan goes up or down in value in line with the value of your home. After 5 years, the value of the boost will change to reflect the current value of your property. This could mean you'd have to pay back more or less than you originally borrowed.
However, the boost is interest-free forever. This means that you'll only need to pay interest on your main mortgage, as usual.
The rates for your main mortgage might be higher than you’re able to get elsewhere – your broker can help you figure out if this is right for you.