so far the layout as follows:-
a6 buy/sell, input b6 amount buy,input
c6 amount sell,input ...

in c6 then i imagine the formula should go something like
=if(a6="sell",(b6-c6), but that does not work for some
reason.

I'd suggest starting with these headings for columns A, B, C.
A1: buy/sell
B1: transaction shares
C1: cumulative shares

Then in C2 put
=IF(A2="buy",B2,"error")
because the first transaction has to be a purchase.

Then in C3 put
=IF(A3="buy",C2+B3, IF(A3="sell", IF(B3C2,"error",C2-B3),""))
which gives a running share count by adding or subtracting as
indicated by column A.

Then copy C3 and paste into C4 downward.

This formula can be used as a model for other calculations of this
nature.


is it possible to have the live date and time displayed
in a selected cell?

=NOW()


anyone aware of portfolio template with more than the
basic features ...?

There are several to choose from he
http://office.microsoft.com/
en-us/templates/results.aspx?qu=excel+portfolio&av=TPL000