Solution

The padlock came with the outboard so I assumed it would be weather-resistant.
Not so, Lidl obviously thought that was too much of a luxury. Rusted solid in three weeks outdoors.
However a Makita battery powered disc grinder solved the problem in 30 seconds.
Outboard now at home for some investigations as to why it won’t tick over and is difficult to get started. The little Suzuki is in its place and did a fine job of getting the boat back to the mooring.