2008 Mini Cooper N12 - P0012 Code - Only 34K miles

Mini threw a P0012 code a month ago, car runs fine otherwise - cleared code - occasionally returns only after cold start.  In an attempt to fix, changed oil - nope.  Then replaced timing chain tensioner bolt, which was not as stiff as the replacement, code P0012 came back a week later.  Replaced both VANOS solenoids, cleared - P0012 came back a week or so later.  Replaced both non-return check valves - 3 days and the code is back.

What's next - valve timing sensors?  I wish the codes were a bit more specific

I have also tried the "master OBDII" reset (leaving both battery terminals off overnight.  It never displays any other code.