Thanks guys.
I ended up purchasing sulphamic acid and silver oxide to make my own Argyrotype sensitizer.
It seemed to go well but I've ended up with a lot of sludge. I've filtered the sensitizer with two layers of unbleached coffee filters but the fine sediment still appears. After sitting a long period the sediment does fall and some light passes through the sensitizer (and brown bottle).
My first thought was that perhaps I didn't keep the sensitizer heated long enough while I was mixing in the FAC ( I recall taking off the heat while adding the silver oxide). So, I reheated it today to 60 degrees C. (as Dr. Ware recommends) and stirred for over an hour (I've got a magnetic stirrer & heater which is fantastic). Anyway, this didn't seem to make any difference, as I've just now re-filtered with the coffee filters and it still has sediment.
I have printed with this sensitizer (with sediment) and it works but the dmax seems a little inconsistent over the coated area.
So, I guess I need to get better lab grade filters as the coffee filters are not working? But where?