If it were me, Emma, I'd freeze it just in case. Expressing can be a lot of work and it is really heartbreaking to see your milk go to waste. Its likely to stay fresh enough in the fridge, but I reckon even so, I'd be doing as gidget suggested and freezing it in 50ml lots. Particularly if your little one hasn't tried it before... You don't know if he'll take it first go, KWIM?
My first go with the hand pump I only got 50ml anyway...