I am thinking of just using my old PVC liner (which actually has another old
butyl liner under it) as underlay for my new butyl liner- would this be OK?
The pond is at the moment quite dirty, would there be any risk that any
beasties left in the old liner would try to burrow through the butyl- i.e.
would it be necessary to take out the old liner, clean it, then replace it?

Thanks,

Will