Discounted Rate
To tempt new customers many lenders will offer a new borrower a discount on their standard variable rate, for a set period.
Your payments will go up and down, as with a standard variable mortgage, but you'll benefit from a discount on the standard variable rate.
After the agreed set period the interest rate will switch into the mortgage lender's usual standard variable rate.
The rate for new borrowers is usually lower than for existing customers. So try to shake off that customer inertia and change mortgage lenders every couple of years - having checked, of course, that there's no penalty for leaving.
The penalties for changing to another mortgage lender may last longer than the agreed discount term.