Jonathan,
Just discovered a box from my old studio containing a new Filinox filter housing with a blue top filter and two boxes of spare grey top (20) filters. I probably picked them up from an old lab or bought them from Process Supplies, Mount Pleasant, London who could offer some information. Silverprint, London may also be of help.
The few b/w printers still operating still have these attached to the mains water inlet.
I intend to sell these on ebay.
The filters would be sold by micron grades. The blue top filter (10 or 15?) included in the housing is not as fine as
the grey top filter, marked 20 on the packaging. Although the filter housing states Made in England the packaging states Assembled in France. Filters are designed to remove all unwanted particles from a mains supply: grit, iron etc.
To generally clean out an in-line filter reverse the water flow. In hard water areas it might be an ideal to immerse the filter in kettle or coffee machine cleaner to remove hard limescale deposits then rinse thoroughly in clean water. Soft water areas have a lesser problem - a thick, clear slime or algae (akin to jelly fish) can accumulate over a long period of time. It can easily removed by rinsing in warm water.
James