Rich
I'd stay away from purlon for use as a ridge line or for stringing up a hammock. From personal experience it's got far too much give in it, even when fixed, stretched & re-tensioned (for my liking).
I got a length of climbing rope / cord that was around 7mm from GO Outdoors which was green with the odd blue fleck in it (not quite as goudy as my other climbing ropes), and you can specify the length you want.
I have subsequently acquired rope from a local climbing centre which has served it's time, the only thing is such rope tends to be around 12mm which if worn can be a lot thicker due to 'fuzz'.
Hope this helps.