Wanna be free

Savingup

Just confused about how overpayments work with nationwide on a repayment have been been overpaying for some years now and have overpayment pot of £14k.
Have been tracking the overpayments on line last couple of months and it seems not all the £100 is coming over the mortgage about £80-90 does but changes on different month spoke to nationwide regard thsi and they say the whole £100 comes of the bottom line,asked the payments stay the same throughout does the below look right to you I have asked them to supply a statement with the monthly overpDate Transactions Paid out Paid in Outstanding Balance:
01 Jan 2012 Opening Outstanding Balance £34751.73
Interest Charged £1862.02
January 12 Payment received £289.44
February 12 Payment received £337.37 Capital and interest @5.63%
March 12 Payment received £337.37
April 12 Payment received £637.37 inc £200 overpayment
May 12 Payment received £337.37
June 12 Payment received £337.37
July 12 Payment received £337.37
August 12 Payment received £337.37
September 12 Payment received £834.37 inc £497 overpayment
October 12 Payment received £687.37 inc £350 overpayment
November 12 Payment received £337.37
December 12 Payment received £337.37
31 Dec 2012 Outstanding Balance £31466.24

01 Jan 2013 Opening Outstanding Balance £31466.24
Interest Charged £902.34
January 13 Payment received £465.37
February 13 Payment received £337.37 Capital and interest @ 5.6%
March 13 Payment received £275.68 Capital and interest@2.5% from this date to May2014
April 13 Payment received £355.68 inc £80 overpayment
May 13 Payment received £325.68 inc £50 overpayment
June 13 Payment received £275.68
July 13 Payment received £275.68
August 13 Payment received £275.68
September 13 Payment received £275.68
October 13 Payment received £326.68 inc £51 overpayment
November 13 Payment received £375.68 inc £100 overpayment
December 13 Payment received £375.68 inc £100 overpayment
31 Dec 2013 Outstanding Balance £28428.04

01 Jan 2014 Opening Outstanding Balance £28428.04
Interest Charged £231.27
January 14 Payment received £355.68 inc £80 overpayment
February 14 Payment received £375.68 inc £100 overpayment
March 14 Payment received £375.68 inc £100 overpayment
April 14 Payment received £375.68 inc £100 overpayment
May 14 Payment received £275.68
02 May 2014 Outstanding Balance £26900.91

Interest rate 2.5% redemption date july 2023 was October 2023 on all previous until asked for a statement of how the capital is paid with overpayments.I have posted before just cant get my head round it though.

Thanks in advance