There was a drip coming from the ball valve in the storage tank so we've replaced it with a floating valve. Hopefully I'll get a good nights sleep? I'll update in the morning!
We can hear a dripping noise (seems worse at night) starts really fast and then tails off, sometimes goes on for 5 minutes, sometimes 10 seconds and at non specific intervals throughout the day and night. We think we've tracked it down to the mains water pipe, as when we turn the stopcock off...