Oops! I can see how that happened - for some reason Gaynor's response didnt come through at the right time - makes the conversation read completely wrong!
The clay soil maybe causing an issue with drainage making the yews susceptible to a disease called Phytophthora.
As your yews are so sick maybe worth the risk of digging up and replanting with plenty of grit and organic matter incorporated into the soil. This will improve drainage- yews need plenty of water to get established but hate to sit in it.
While dug up inspect the yews rootball to see if there is any sign of new growth. Re-planting with mycorrhizal fungi helps to establish a healthy root system quickly.
Unfortunately you are up against it as yews can be hard to get established. Does the owner have a dog? Seems a strange question but a client of mine had the same problem as yours. I tried everything with no success until I found out it was because the yews had become the dog's favourite peeing post! Sadly it was too late to save them and we planted a fantastic holly hedge instead.
Sounds like they were dodgy in the first place, but also the clay soil and drainage is a problem. Have they any roots when you dig up the dead ones?
I used to work in a garden with truly horrible drainage and the clients had already planted a few years before I started there. We'd take out the the bad looking ones, transplant them elsewhere onto better drained soil and they'd come back, but you could never replant them back, they'd just die immediately. The soil literally was stinking in places. I left that garden a few years ago and I think they've ripped them all out now, and they were lining four large parterres...
See this article: http://www.terrainaeration.co.uk/2009/01/how-to-treat-phytophthora-...
Lynda Green of Terrain Aeration is a member here: http://www.landscapejuicenetwork.com/profile/LyndaGreen