Our 20 Litre Outdoor Solar Shower leaks if a hose isn't constantly attached, why?

The hose connection at the bottom of the 20 Litre Outdoor Solar Shower is lacking a one-way valve, which is why it leaks when not connected. This behaviour is expected for this particular design. The most effective solution to prevent the leak is to install a permanent section of hose that includes either a shut-off valve or a non-return valve. This will ensure that water doesn't escape from the connection point, thereby eliminating the leak.