Hi Andrew,
There seem to be looms of very varying quality out there these days, I suspect the Indians and Chinese are getting in on the act!!
Like many here I also make my own. There are 3 choices, copy the original wiring diagram, copy what's already there or make what is actually required..
I tend to go for the third option whilst keeping an eye on the wring diagram. Original wiring looms were often made to suit more than one model and have redundant wires..
Buy decent quality cable and connectors. I go for brass bullet connectors but always solder them, shrink-wrap is freely available to tidy things up. Being a pedant I often run more earth wires than were originally included, especially one from the headlamp area direct to the main earthing point.. Even then you'll be surprised how few wires are actually needed!!
Hope this helps, good luck..
Adrian