There is currently some confusion over the Nilfisk suction kits and filters for their washers. The Nilfisk web site lists a Water filter 106411231, a Water Suction Set 106411232 and a Water Suction Kit 126411387. Unfortunately there are no pictures provided so I cannot be sure which is which.
To add to the confusion the Nilfisk manual does specifically recommend the Water Suction Set quoting part no. 126411387 ( ie the Kit according to their web site and not the Set ) to suck water up from a tank or butt to supply the washer instead of from the mains, but it does not tell you how to do this. In the manual it just says "place the other end of a hose ( apparently any hose up to 5m long ) in a water tank at the same level as the washer", so an ordinary garden hose would seem to do the job just as well.
If the washer is already supplied with mains water then this accessory can be used to empty ponds etc using the bernoulli principle, but you need to have somewhere nearby for the water to drain away as the hose is only around 5 feet long.
Ironically, my suction kit came with a bayonet to Click & Clean adapter 126411389 which is currently advertised separately on Amazon for £32.51, so if you need the adapter anyway you are getting the suction kit for free - but check first that it is included in this offer.