You're looking for a nano (so you may find tweezers helpful for extracting the log), nearby to a modernistic looking dovecote.
I'm including this in the MK Artwalks series even though it's not listed on the website like the others. The cache is NOT on the dovecote itself - when you get there you'll see why! There's plenty of parking here, and nearby you can also see what is apparently "the largest purpose-built Indian restaurant in Europe".