Looking at NFM06, using the following;

ORIGIN : Boundary Zone 5 (0044)
DESTINATION : Gatwick Airport (5416 / GTW)
DISCOUNT : Network Gold Card (NGC)

Gives the following.

£9.40 - (SOR - Standard Anytime Return )
£7.40 - (SDR - Anytime Day Return)
£5.15 - (CDR - Off-peak Day Return)
£4.75 - (SDS - Anytime Day Single)

As the OP is travelling out one day and returning a couple of days
later, would the SOR fare cover both journeys (as there's no mention
of the word 'day' in the title)? If not, I suppose 2xSDS would need
to be used, giving a total cost of £9.50.

Rdgs

M