Hi Graham and Helen
Yes, there is a screw going through the bottom of the stanchion base, through the base of the toe rail and the deck, into the wooden batten which runs all the way along underneath the deck.
Unless you can separate the stanchion from the base there is no way of accessing this screw. My stanchion was firmly welded to the base so I ended up removing the 2 bolts securing the base to the toe rail and then rocking the stanchion back and forth until the screw broke. Hopefully, there will be enough screw left to get a pair of mole grips onto it to then remove it. As I was replacing the stanchion and base with new ones I was able to put a new screw. The alternative would have been to double bolt the base to the upright part of the toe rail.
I hope this helps