not a mistake groundhog its a choice , they would'nt be making trolley's if there was'nt a market for them, if you look at martins initial post he does'nt want a tank in the back of his classic VW,
martin i us a water 280 ltr water but for outside storage , you can buy a jet pump or something similar from local aquatics shop for about Ł20-30:00 a bit extra for hose and you can transfer water quite quickly , i can fill 10 containers in less than 10 minutes, hope this helps
also when you get a brush , you can fit it with 1mm pencil jets , which gives a higher pressure rinse with much lower flow rate than a brush with 2 or 3mm jets you can get the 1mm jets from cleantech