Because the other side tail light is going, then the socket on the car must be ok.
If they are pre-wired sealed units then find where the two join up and check the terminations. If you are sure you have voltage going to the cluster, then it may well be faulty. It might just be a coincidence that it worked again.
Basically start from the plug and work your way out to the lights checking for voltage or swapping the tail light wires from one side to the other to prove what is faulty.
Hope that makes sense.

Edit- just read that the light on the mudguard is not going as well. This comes on with the tail light so picking it’s a connection/joint issue.

Have you undone cover and checked wires into block connector in plug to check they are all connected and not corroded?

Grasshoppa wrote: Presume there's no issues with other trailers ruling out the plug on truck?

Interesting you mention it's a Voyager trailer.  I had to replace the lights on my Voyager trailer as there were problems.  Turned out the LED lights were actually fine, but they were not very well wired up.  Definitely sounds like you also have a wiring problem. I replaced them with a set that had wires from the lights to the plug without joins.

Take a little flat screwdriver and open up the gap on the pins in the plug - they tend to close up after a bit of time and you start to see these sorts of intermittent faults. 

Thanks for all the input, team. As someone with no clue about this kind of stuff, I appreciate it.

Glad you located the problem.