I seem to have messed up and missed some, but i have no idea how many i've checked the pip boy stats but can't see anyway of finding out how many i've detonated unless i try and find and detonate another.

Really awkward not knowing how many i have missed

In the notes tab press R to view challenges. That one's called Warhead Hunter.

it's not showing on my PC version

you have 2 tabes general and reputation it's not in either of them ???

Oh it's not in my notes tab either All that is showing is the new GRA challenges?