Paying Mortgage Capital with Cash ISA's

Hello, im a newbie....

I've have posted this question in frustration of my mortgage provider, C&G

I have had an interest free mortgage for 4 years, 2 years at 0.89% and the last 2 years at 2.5%
Total borrowing is 270K over 25years

Here's the question that i hope someone will be able to answer:

"if i were on a repayment mortgage at the same rates, what would my total amount of capital repaid been over the last 4 years?"

Now, I'm not a fanatical wiz or anything but i find it difficult to understand why C&G cant give me a figure?
I did a "back of fag packet calculation" and i make it around 40k, how far out am i??

I buy 2 max cash ISA's a year to repay the mortgage capital, I'm self employed so this helps me if for any reason i need cash quickly, at the moment i have not had to touch any of the ISA's, and i believe its a better way of paying the mortgage rather than simply "giving" the money back to the lender never to see it again until you "downsize"?

Any financial wizz's out there?!

cheers marcus
247 (my first 3)