Thanks everyone for your help.

What I ended out doing was putting general stats at the top of the
spreadsheet; years, number of periods, periodic interest rate, present value,
fortnightly payment.

D1 years
D2 d1*26 (for fortnightly)
D3 interest rate
D4 NEGATIVE value of mortage eg -$100,000
D5 (((D4*-1)/D1/26)+((D4*-1)*D3/365*14))

To work the payments and interest calculations, I haven't stuck to the true
rules of charging the interest on the first day of the month following.
However, I think what I have done is fairly close to reality, and good enough
for decision making purposes. In fact when it comes to drawing down the loan,
I will to a far more detailed spreadsheet with each day per row (I still
can't think of a way of automating this part).

Anyway, in the calculation section I have; period #, total payment, interest
payment, principal payment, extra payments, and balance of loan.

A10 = 0
F10 =d$4*-1

A11 = a10+1
B11 =sum(c11:e11)
C11 =(d$4*-1)*d$3/365*14
D11 =(d$4*-1)/d$1/26
F11 =f10-d11-e11

A12 =a11+1
B12 =sum(c12:e12)
C12 =F11*D$3/365*14
D12 =d$11
F12 =f11-d12-e12

I think copied row 12 downwards to accumulate multiple fornightly payments.

I hope that this helps other people.

Cheers, Ty