The harbour master has looked at it. It is a brick build structure 5.1 by 3.6 metres and was probably a pump house originally.
Currently it is a substation according to the big yellow sign on the door (and the ones warning of danger of death on all sides). Ref no 93657. He has spoken to UK power networks who also agree it's a substation but they were not sure if it is disused or not. Regardless they were going to contact their estates department to send someone out to survey it and do any maintenance required. As soon as they come back to the harbour master he will contact you.