Cleaned the contacts and Vaselined them to inhibit corrosion.
Sprayed the socket and matching plug with contact cleaner.
Refitted and discovered that the plug hadn't been fully clipped in place - it pulled out very easily but should have been retained by the spring clip seen in picture 2.
Also the positive high current connector screw was loose on the relay body - properly tightened up now.
He's going to work at five and will chuck his bike in the back in case it fails to start at one o'clock in the morning.
Worked fine today though, typically.

Rubbish photo's, sorry.