If you've got a Facebook account I'm am member if this group
https://www.facebook.com/groups/3222...?ref=bookmarks
PC and pc parts for sale UK
They do have a sales page but also a general page and people post all the time asking for general help. it's a pretty active group. they maybe able to swing some answers your way.
I'm running a 4690k stock speeds with a H100i v2 AIO cooler and my idle temps are around 25/28c apart from summer temps at the moment its sitting around 34c. thats on slow fan speeds though.
I know AMD chips run hotter but 51c it above normal.
What case is it sat in and have you checked to see if the fan is mounted right?