That's what we've been doing for the last 3 years.
Committed to interest only, but put away an extra €350pm into a high interest savings account. At the end of the year, we'd throw the lump sum at the mortgage thus reducing the principal.
But to make it worth while, we kept the repayments the same, but added the difference to the capital repayment, as well as adding another 5% onto it.
That might sound all over the place, but as an example....
Our interest was costing us €1000 per month.
Our savings were €350 per month.
After 12 months, we withdrew the €4200 (€350x12) from the savings, and paid it off the principal.
Following year, interest was costing us €950. So we added the €50 to our savings, giving us €400 per month to save. We also added an extra 5% onto it, just to keep ahead of the game. So our monthly savings were €420 per month.
After 12 months again, we withdrew the €5040 (€420x12) from the savings, and paid it off the principal.
Following year again, interest dropped to €900. We added the €50 to our savings again, giving us €470 per month to save. We added another 5% onto it, giving us a monthly saving of €495.
And on it goes.
Obviously that's a simplistic example, but it gives an outline of what we did. It doesn't take into account the interest you'd earn on the savings, but we were earning 6.5%.
It's also good in the way that you're only committed to paying the lowest amount possible for your mortgage, so if things are tight some month, you don't have to put away the savings for that month.
Oh yeah, and there's no penalty for paying money off the principal as long as you're on a variable rate.