Forgotten Book: A LANDSCAPE OF LIES, Peter Watson

This contribution to Pattinase's Friday's Forgotten Books comes from my records in 1991.

Fantastic Fiction has a list of 5 novels published by Peter Watson, intellectual historian, 1985-1995.

• The Nazi's Wife (1985)
• Crusade (1987)
• A Landscape of Lies (1989)
• Stones of Treason (1991)
• Capo (1995)

From Wikipedia, it appears he has written a number of academic books as well. 

Publisher's Synopsis
Isobel Sadler is dead broke, and the only thing left that might bring in any money is a stupendously bad painting that’s been in her family for generations. It’s so ugly she can’t imagine it would be worth much...until someone tries to steal it.

Mystified, she turns for advice to art dealer Michael Whiting, who identifies the painting as a 16th-century treasure map, pointing the way to a cache of priceless religious artifacts that were hidden by monks when Henry VIII was dissolving the monasteries. If he and Isobel can decipher the clues in the painting, Whiting reasons, her money troubles will be history. But if they can’t decode the painting quickly, Whiting and Isobel could be history themselves
.
Even as they struggle to translate the arcane instructions — laced with references to everything from the Bible to Botticelli — a rival is dogging their trail, and he’ll stop at nothing, even murder, to get his hands on the medieval gold.

Another review had this to say
Before there was The Da Vinci Code, there was Landscape of Lies. A more intelligent, less sensational, less riddled with religious quackery version of the famous best-seller, Landscape also seeks a hidden treasure, its hiding place to be discovered by deciphering the clues in a painting.

LANDSCAPE OF LIES sounds intriguing:

From another reviewer:

Landscape of Lies, by Peter Watson, opens with a painted scene, literally - the inside of the cover features a fold-out image of the bizarre medieval landscape that’s at the center of the narrative.
The story begins with the painting as the target of a failed burglary attempt. The painting’s value is not in its fame (it’s obscure) or its beauty (it’s mediocre), but in its secrets.
In essence it’s a treasure map. Each element, figure or object, placement or color, carries a symbolism that, if decoded correctly, will lead to a cache of religious artifacts hidden by monks during the rule of Henry VIII. From city galleries to ancient churchyards, the chase is on!
Despite the sometimes silly dialogue and the predictable romance, the story is a clever one. Originally published in the UK in 1989 (four years before the deciphering of the DaVinci Code), this title was published by Felony & Mayhem in 2005.