I apologize for my short-tempered response in #10; I was evidently not myself so late at night, but I shouldn't have been so rude. I think I was thinking, if anything, that the BBC site from which that sentence was first quoted should have been more careful. But yes, I do disagree with penguin and hb in this instance, depending on context, and I tried my best to explain why, using examples.
I understand that it's evidently confusing to many people, as this general area of singular-plural agreement has given rise to a lot of discussion over the years in the forum. But for those who still have questions, I can only recommend again the link I posted in #7, which itself has further links, some of which may still exist. Or perhaps dirk or Martin would like to try to help explain.
Part of the problem may be that 'average' is simply not as frequently used as an expression of quantity, since there are usually other ways to phrase statements about approximate numbers, some of which I also gave among the examples. But in this case, it seems clear (to me at least) that when 'An average of' is used as an expression of quantity, it behaves in the same way as other expressions such as 'A number of,' 'A total of,' 'A variety of,' etc.
Someone like MikeE or sebastianW might have been able to cite a grammar book to help clarify, but I'm sorry, I don't have one at hand myself. (Speaking of missing LEOnids, I'm a little concerned that I haven't seen either of them around in a while.)