Sorry to hear about your troubles Sue.
It shouldn't be a hard process to change to interest only. I'd imagine the best course of action for yourselves would be...
Ring your bank.
Explain the situation.
You haven't mentioned that you've considered this part but, tell them that you want to extend the term of your mortgage to the maximum allowed given your age, and tell them that you want to go interest only.
They should send you out the forms to get everything "official".
You'll then have to get your life assurance renewed to take into account the new term of the mortgage.
Get a copy of the new life assurance policy, and send it back to them with the forms they gave you.
Everything should be changed, and hopefully you get back above the breadline.
The only problem with the above is that you might have to forfeit your tracker.
But a tracker's not gonna be much good to you if you lose your house, God forbid.