Outstanding Balance owed Questions

Mark-W_2

I have a question regarding how the existing balance is calculated on a mortgage.

I have a mortgage with the Halifax that is currently in 2 parts both are repayments, Part A fixed at 4.99% until 1st Jan 09 and Part B SVR at 7.00 (6.5% as of 1st nov).(This was also a fixed rate of 4.99% which came to an end recently)

I phoned halifax to get my current balance which was broken down by Part A and Part B.
Part B had fallen from around £77k to £72k in line with the repayments.
Howerver Part A was quoted as having roughly £73k outstanding, the exact same amount as when the mortgage was taken out.

How is the existing balance calculated?
Why hasnt the outstanding balance been reduced in line with my repayments?
Are the repayments only taken into account when the fixed rate comes to an end?

Thanks for any replies.

Mark.