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First, make sure it really is the left hand cotter that's loose - if the r. hand one is loose you will feel it through the left hand crank.
Whichever one it is, check that the spindle hasn't damaged the cotter pin - if it has, file the pin flat and try again. The pins are much softer than the spindle, so no need to worry about that being damaged.
When fitting the pins it's important to support the spindle and give the pin a sharp tap with a hammer. I like to put oil on the whole assembly, just to help the pin into position. Retighten the retaining nut.
Now check the cranks are at 180 degrees!
After your first ride, give the pins another little tap and retighten the retaining nuts - not too much force, remember they're quite soft.
I doubt if you will have any more trouble after this treatment.
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clubman
@jeff80
Having a cotter pin crank issue on my Paragon. The non drive side crank keeps working a little loose on the bottom bracket spindle, not so much it comes off but enough that I can feel a worrysome play in it as I pedal. The crank is an old BSA double fluted and the BB a Baylis Wiley hollow type. I've already replaced the cotter pin with newer ones but after a month or so riding it has come loose again. The bike is set up as fixed and is currently being pressed into use as a wet weather commuter due to a broken frame, so is seeing more miles than it usually would. Could it be, as I fear, a worn crank or BB spindle, or is there another possibility to investigate first?