Will be interested to hear of more detail on this. The marina is not open to the public, so the implication is that it was a boat owner or a member of the fitness center (bit late for that?).
There are also very few places where it is possible to get access to the river from the marina unless you can fly over a substantial fence.
I'm not argueing, merely asking. The article says "off Gillingham Marina" could that be outside? I mean, how does one access those tidal mud moorings outside the lock?