Reinstalled the pump on the engine
today and gave it a test run. All good. Engine
runs at full power and doesn't piss fuel
out of the drain hole on the underside of the pump. Works just like a bought one!
If anyone has a similar problem, I'd encourage you to have a go at fixing it with a new membrane. Here's the Australian supplier I bought the diaphragm material from (not ebay as I previously stated. I was confusing it with another purchase):