An update:
I noticed when I was tweaking it last night, there is some resistance from the chainring/crank when turning them.
If I just use my little finger to gently push the crank arm, you can feel it ever so lightly resisting/relaxing with each chain link that engages (or released) from the chainring teeth.
A kind of (and I stress, the slightest of slight) judder as you slowly turn it - again, I'm positive this isn't the BB.
As above, is it possible to over tension the chain with a chain tensioner (if it's not at one extreme or the other and there is decent movement in the chain)?