There are a heap of financial functions built into excel. Look up PV (present
value) in Help and also refer to the 'See also' which lists the other
financial functions. I am sure you will find something there to help you.

Before developing the formula, there is a more fundamental question to
be answered.  The problem is somewhat analogous to the mark-up /
mark-down discount issue.   If you buy an item for $1.00 (sorry my
computer does not have the pound sterling symbol) and you sell it for
$1.25, have you made 25% profit, or is the profit on the sale 20% (25/125)?

The way you did your computation, you took the original amount, $300,000
and said that the impact of 6% inflation would be to increase it to
$318,000.  You then multiplied that amount by 0.97 to get an inflation
adjusted value of $308460.

In as much as the interest is being accumulated over an entire year, and
 inflation is also a gradual process, I would think that the inflation
adjusted value of an investment paying 6% after one year where the
currency is experiencing a 3% rate, would me merely 3% more than the
original investment.  Thus, I would say that the inflation adjusted
value of the investment after one year would be $309,000.

Once you decide how you want to compute the inflation adjusted interest,
writing the formulas is quite simple.

Pick a cell, perhaps cell B1 and place the interest rate in that cell.
Pick another cell, perhaps cell C1 and place the inflation rate in that
cell.
Place the funds on deposit in cell A1.

Place this formula in cell A2,   =A1*(B$1 - C$1) to compute the
inflation adjusted value after one year.

To compute the value in subsequent years, copy the formula down column A.

Somewhat admittedly off topic... A much much much tougher question to
answer is how is the United States going to keep both interest rates and
inflation low, so home owners aren't squeezed out of their house and
foreign governments don't abandoning investing in US dollars.

WindsurferLA.