If anyone is interested I've done some more digging and found a couple of old press releases on the Pizza Hut website that would indicate that the cheese blend is approximately:
Mozzarella (23%) / Provolone (23%) / Cheddar (23%) / Monterey Jack (23%)
Parmesan (4%) / Romano (4%)
I just tried it out on a thin and crispy pizza and it tastes pretty good to me. I would suggest that you need to use a pretty plain tomato sauce to really appreciate the cheese flavours. It might not be to everyones taste but I enjoy a really cheesey taste.
Can anyone recomend other interesting blends?