Automated transactions account total scope

[MF]

Hi

I am trying to setup an automated transaction that will allow me to 
manually add a simple transaction for a mortgage payment and the automated 
transaction will split it out to add an expense for the interest and reduce 
the liability with the remainder. Everything works for the first month, but 
after than, accessing account("Liabilities:Home Mortgage").total in the 
automated transaction seems to be giving an incorrect value.

Here is a demo file:

define mortgage_interest_rate = 4.0 / 100 / 12
define mortgage_interest() = (-account("Liabilities:Home 
Mortgage").total)*mortgage_interest_rate)
define mortgage_principal_addition(payment_amount) = payment_amount - 
mortgage_interest()

= Expenses:Mortgage
  Expenses:Interest:Mortgage Interest   (mortgage_interest())
  Liabilities:Home Mortgage             
(mortgage_principal_addition(amount))
  Expenses:Mortgage                     (-amount)


# Opening balances
2012/08/01 Initial mortgage liability
  Equity:Opening Balances  £100000.00
  Liabilities:Home Mortgage


# Mortgage payments
2012/09/01 Mortgage payment
  Expenses:Mortgage        £1000
  Assets:Checking

2012/10/01 mortgage payment
  Expenses:mortgage        £1000
  Assets:checking

2012/11/01 mortgage payment
  Expenses:mortgage        £1000
  Assets:checking

2012/12/01 mortgage payment
  Expenses:mortgage        £1000
  Assets:checking

2013/01/01 mortgage payment
  Expenses:mortgage        £1000
  Assets:checking

Then running the command 'ledger reg interest' gives the output:

12-Sep-01 Mortgage payment      ..In:Mortgage Interest      £333.33     
 £333.33
12-Oct-01 mortgage payment      ..In:Mortgage Interest      £666.67     
£1000.00
12-Nov-01 mortgage payment      ..In:Mortgage Interest     £1000.00     
£2000.00
12-Dec-01 mortgage payment      ..In:Mortgage Interest     £1333.33     
£3333.33
13-Jan-01 mortgage payment      ..In:Mortgage Interest     £1666.67     
£5000.00


As you can see, the first month appears correct but the values should go 
down slowly over time because the liability is going down. Calling 
'account("Liabilities:Home Mortgage").total' in the automated transaction 
seems to be returning £100000 in the first transaction (correctly) but in 
the second and third transactions returns £200000 then £300000 etc.

Is what I am trying to do possible?

Thanks in advance.
MF