Don't need a pump provided there is at least 3 feet of height for
gravity feed. Did this with brother-in-laws Commercial Steam Cleaner
(45 gal plastic butt on top of 45 gal drum) for washing his artic
lorries. will was 3 lorries WITH trailers before running close to dry.

You MUST filter it though - preferrably twice - as it goes into the
butt to stop big leaves, twigs, dead birds etc, and the mesh from an
old kitchen sieve (or even a cheap new one) over the outlet from the
butt that feeds the washer.

Don't have hosepipe ban problem in sunny??? NE Scotland - just
commercial water meters and charges to remrtgage for!!