There are tons of them all over the place. My mate in work digs for bottles at sites of them. The bottle digging community are a secretive bunch and only reveal the where abouts of them after they can no longer be dug at. I do know theres one in Prenton by the flyover where the old pond used to be(they filled it with rubbish). There is also one under the new estate behind Somerville health centre on Gorsey lane Wallesy. I was working there when they where digging the foundations to the houses. You could literally see the layers of ceramic and glass in the trench walls. My mate (the bottle digger) was like a dog with two dicks.
Part of Sefton park is over one too I am lead believe.