Update:
I recently placed an order with Titan Reloading for a heap of reloading stuff, so it seemed an opportune time to order another .490 mold as it only added a couple of $ to the overall shipping cost. (After not receiving any response from my letter to John Lee, I've come to the conclusion that a) I'm not going to get any help from them, b) they're very inconsistent with their customer support policy, c) they're selective in who they decide to help and d) John Lee couldn't care two hoots about his customers, unlike his father).
So, with some trepidation I cast a few balls with the new mold. After letting them cool, I started measuring. First reading was .485 - urk, not good - so I moved around the circumference, .485 - .487. A bit undersize, but at least it's not out-of-round. Tried a few more and found that some agreed with the first readings while others were up to .488. It appears that the two cavities are fractionally different. Still, under-size I can work with.
It was a relief, because I'll admit that I wasn't too confidant to start with.
Oh, and the funny thing was this parcel had been opened by Customs, first time that's happened to me. I'm guessing it's to do with the fact we're on "heightened alert", and I suspect that anything to do with firearms is automatically suspect.