Also, if you mirror J16, J17 and J18 and place them close to U1, then the wiring is a lot easier to follow.
AS drawn the 3 wires meeting at the regulator output.
Are you sure Kicad has actually connected these to the regulator?
This is less ambiguous:
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I think if you place a PWR_FLAG on your drawing and illustrate it so its clearly seen as a separate symbol in the KICAD library that will solve the whole issue and you have the 5v in 3.3v out shown correctly. I havenāt actually seen one (PWR_FLAG) on any of the schematics yet and I think that was the initial error in the first place 
This is geoās first go after all and will get the hang of labels and flags in time, unless Iāve lost the plot somewhere 
The power flag is shown on one of the diagrams above. However, I have just tried removing the 3v3 power flag and the problem seems to have gone. ( no errors on the ERC)
Am I correct in saying that a power flag is only an external input to a circuit? If, in this case, the 3v3 is produced on the board via the regulator, it should not have a power flag?
Correct.
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Once more - it is not power flag. It is a symbol of connecting to 3v3 net.
Yes and no.
You have to understand what ERC can check not understanding the schematic.
It simply looks - O! here we have some Power Input pins connected to one net (= set of wires connected together), and we have the IC pin named Power Output also connected to that net - everything ok.
But if you have 3V3 produced on the board but routed through some filter (ferrite bead for example) then after this filer there is new net and if there will be Power Input pins connected to that net ERC will be seeing Power Inputs connected to net and no Power Output connected to it = error.
In such case you will have to use PWR_FLAG (Iām not sure of name as I donāt use it) even your power is generated on board.
Any element in the power line (for example small value resistor used to avoid unstability of LowDrop) make ERC not knowing that power goes from one its side to the other.
None of the pictures above show the correct use of the PWR_FLAG symbol and this is part of why you canāt understand. I havenāt yet learnt to post images otherwise I would show you. The voltage designators 3.3v, 5v, -12v, 12v, ect. dont matter for KICAD it doesnāt care, they are for you and me to read and understand the circuit. Kicad does not need a value but it must be told where the power points are and this is indicated with a PWR_FLAG in conjunction with the desired reference, you need both, in this case 3.3v and ground. mikethewireās circuit above is correct except there are no PWR_FLAG symbols shown on it on it. 
Very easy, copy an image and paste it, or drag a file from a file manager. Or use the Upload button above this text editor.
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