when I was a gas fitter in the late 60's we used hemp pulled off a hank, like loose hair rather than string. Used with common plumbers jointing compound it worked well on tapered(BSP) threads holding water, but was less successfull on parallel threads, they usually needed a flange or locknut to close onto.
The bathroom waste is not a good model, uses parallel threads but holds no real pressure in use.
The fork leg threads are parallel and fine thread. I would now use PTFE wound clockwise to tighten with the seal holder, but as the threads are so fine I would only use maybe three turns, if you over do it you could end up cross threaded.Put a thin smear of silicone over the tape that will fix just about anything unless the thread is distorted.
a good home made tool is well worth having, the actual tool is available from Drags for about twenty quid plus the usual. If you dont have one you end up with pipe gripper marks on the seal holder ( like mine) and also run the risk of a distorted fork leg also like mine....I should send this note to the previous owner..