Battery drain in passive mode: Some East UK preamps draw power from the battery even in passive mode. This is a design feature to avoid loud thumps when switching between modes, but it means the battery will drain regardless of the selected mode.
Make sure the black battery lead terminal has only one wire connected.
Always remove the jack plug from the instrument when not in use, as inserting the jack activates the battery connection.
Use a new, high-quality battery for best performance.
Check that the jack socket is not touching any wiring outside of the preamp circuit, as this can cause unnecessary power drain.