Neither do I know nor care what is near, or close, quantitatively. I would think that directly under the supergrid is a definite no-no.
I have read many reports and heard some also (but rarely 'first hand'), of tetchy bees when under power cables. We now know, after much fudging by the CEGB) that power line emissions/fields are almost certainly the cause of some childhood leukemia cases. So there are effects to living cells and organisms. Bees are organisms, just like us.
I know not what the voltage was, to which poly Hive was referring. I would consider a 33kV supply marginal, but I avoid any risks by not placing bees directly under the supergrid, or any high tension power lines, and reduce the effects, if there are any, to a minimum, by site choice, in any and every particular circumstance.
I have bees fairly close to a 3.3kV supply, but am not worried too much about that.
Whether the effects might simply be on temperament (cf impending electrical storms) or of productivity, I have not a clue. And I don't intend finding out!
Regards, RAB