Mine was running cool too and I suspected a 'cover up' during purchase but I still brought it, too much in love I think..
Turned out a stat was in place just failed open. I got a facet one because it was a brand I knew and it had the bypass hole in it.
If it's not got the bypass in it I'll return it whilst I'm still there. I guess I'll test it in boiling water prior to fitting, otherwise what else can you do!