On 31 Mar, 18:24, Ed ex@directory wrote:
The first water barrel (A) *on my allotment is at the top of a slight
slope and is fed by rain water gathered from the roof of my shed. * I
want to put in a *second barrel (B) in a direct line

Taps near the bottom of each. Linking pipe between both that's as low
as possible, but no closer to the bottom than 6" of either (allowing
for crud and for barrel curvature). It doesn't matter if this is
horizontal, downhill or uphill.

Wickes sell all the bits (look in gardens), cheap too.