LED Indicators

They do not mention what the polarity of earth is, but it is most likely Negative (Nortons are Positive) not insurmountable. I doubt very much they comply with Construction and usage ie are almost 'illegal' but then do we worry about such trivia? 
Over to you.

.. how visible these would be especially from the rear. As Al says, polarity shouldn't be a problem. You'd keep them isolated from the bike's systems and switch the earth (positive) side with the negative of the device plugged into the bike's live side. Unless of course you're already negative earth.

They don't look as if they have an indicator switch.  And how exactly are they powered?  LED need current control through a resistor.  Are they complete?

During springtime, I purchased a basic indicator kit online for £40. It did not come with the indicator stalks but was supplied with a handlebar light switch and horn unit. I was able to purchase the stalks in a version that suited my bike from another supplier. These had filament bulbs so there was no conflict of polarity.

The Indicator kit had all the wiring cables and plugs so was really easy to fit. Mounting the stalks taking the most time as I had to manufacture P clips and brackets. I fed the power cables direct to the battery via a fuse so that my Norton Positive earth wiring was not conflicting with the Indicator unit. I also added the horn to the new switchgear via its own wiring.
Then set-up works really well and allows me always keep both hands on my clip-ons when manoeuvring.  After a few weeks of use, I converted all the indicator bulbs to LED's.
The gear has been 100% reliable, does not drain the battery and does it job well.