My venture into WFP was with a trolley system from Peter Fogwill some....what...5 years or so ago.
Operated it out of the back of my Toyota Corolla, got on perfectly well with it, but operating WFP out of the back of a car is hard work, especially lugging around 25L containers of water and the like.
So it gradually mutated into a mini van mount setup and the trolley itself slowly became redundant.
Then along came the van and I went fully van mounted...great!
My new partner now works with me, when she started doing so I equipped her with the window cleaning warehouse backpack.
Started out as a straight forward backpack (never worn on the back though!!) which soon mutated into being carried around on a stripped down shopping trolley (much like the one pictured above).
But with heavy use the batteries on these things don't last long, the supplied chargers are pants too, so I soon ended out adding a small car battery, but these shopping trolley's are pretty flimsy things and ultimately the weight was too much...wheels fell off, the bloody thing would tip over! and so on.
So I bought a collapsible sack truck and bungee strapped the lot on there!!
Best thing I've done, it is a fantastically versatile piece of kit now, would be no good used out of the back of a car, the weight is too much really, but out of the back of a van it is safe and straight forward, all lifting can be done correctly.
The trolley is narrow and so far have had not a single problem getting around the back of anything, even through houses! (not had to wheel it over carpet as of yet).
To fill it I have a transfer pump in the tank in the van, takes 20 seconds or so to fill.
One speed pump I know, but I just have a very simple inline tap to regulate the flow, just the same as I do with the van mount.
I adjust it to about a litre per min and that suits me fine, on domestic houses I can do 2 standard size 3 bed semi's per fill.
Used alongside the full man mount the two of us can work very efficiently indeed.
The WCW backpack is a really good one, little to go wrong and ours has been going flat out for the last 8 months.
Hated the charger on on it so I bypassed it and use an intelligent charger on it instead, though as said, I now use a small car battery, but I had done this anyway prior to cutting out the original battery.
Ian