As the guys say 18mm manila is the best option but tbh is quite difficult to get & more expensive than the readily available blue nylon which may not be as good on the hands & is not susseptable to rot or chem damage @£100 per roll good value.
as for the complience cert we do our own as under lola & so by law it just states a competant person, there is no qualification to deem this basic bit of kit fit for use, its inspected for cracks/damage the spindel is regreased and a new split pin put in& a load of 150kg (over required cert) is suspended for an hour to check its ability on a long term suspended load -its then repainted its serial number logged & it goes into stock ready to use again. A new gini is about £30 so if you only want the odd one now & again just buy it, but ive got quite a few & like to save my stock