First off at the moment changing to interest only isn't going to make any differenceas has been pointed out to you already.
I would point out just because your wifes sister saved £250 by going interest only doesn't mean everyone will.

Secondly, no you're not going to be able to change as chances are your income will no longer suffice for you to change.


If I were you I would fix your deal again sooner rather than later as when interest rates go back up you will be screwed. While you're at it changing to interest only will fix your payments at a similar level to now as if I have read correctly the change to interest only offers you no savings.

J