TOPIC:

  Going "contactless" is not all it's cranked up to be... ,  well it was basically a non starter for me?...  Sorry...

I hope it isn't a cold solder joint. Luckily it didn't pop off while I moved things about .  I didn't know if I should re-solder that oxidised connection, old wires always seem so horrible to solder. If it happens again I might give it a go though.

I tend to agree with Scotch here. Oxidation noted on the black wire is indicative of an issue.
Some quick soldering tips-
Cut /clean the wires back to new copper. It might cost you 1/2" of wire.
Tin the wire end and the contact your about to solder the wire to.
Solder the 2 parts together with the heat from a pencil soldering iron.
Once your done and things are cooled, inspect your joint and if you're happy, 
Neutralize the completed joint with either and mixture of baking soda and water or use a can of battery terminal cleaner. Both are the same composition. Resin core electrical solder has an acid flux to clean the copper of oxides and dirt and float the contaminants to the surface of the joint. If you leave the flux on the joint (like most people do) you risk corrosion of the joint when moisture from humidity/rain comes in contact with the joint. Baking soda/water mixture is basic and will neutralize the acid in the flux.
You can follow this up with either a smear of dielectric grease or what I prefer, Rust Check or Krown rust preventative spray. It leaves a light coating of ( I believe) soy bean oil. I treat all my electrical contacts with usually Rust Check and have never had a problem. Krown rust prevention spray is rated for 40,000 volts dielectric .The neutralizing step is what some may believe is above and beyond, but I do it religiously since my millwright training at the nuclear power plant.

Thanks for the detailed info Kawboy!  I would like to sort the oxidised wire for sure, I'm worried I might open a can of worms.  There wasn't much spare wire at the switch area, I'll have to see If I could pull more through, but the multi connector on the other end didn't have much spare from memory.

I nearly broke my back pulling the tank off last time due to leaving my tank full, to avoid corrosion from Ethanol.   I'll use some of the fuel first this time.