MTed, here's the information on the cryptic gardens source.
"The Pursuit of Paradise: a Social History of Gardens and Gardening" Jane Brown, Harper Collins, 1999, ISBN 0 00 255844 0
There's a chapter called "Secret Gardens" which starts with medieval monastic gardens, passing those in nunneries which may have had Marian plantings, through the Catholic gardens of the Tudor period and on to others later. It does acknowledge that it is difficult to find traces of these secret gardens because of the conditions at the time. The radio program went into more detail than the book about Thomas Tresham, and I only partly recall his history. All that the book says is that he carefully planned the garden at his home Lyveden (that doesn't look a correct spelling), and that there has been some garden archeology there. It also refers to the priest John Gerard, who used these gardens to hide, disguised as a gardener.