Petplan do a big discount for your first year, and then get you with a rise when your renewal comes through! I've just reinsured with NFU and am saving a significant amount, plus have been guarenteed that it will go down next year.
I think it depends on the type of horse. Petplan are a lot cheaper for my section A than NFU were, but were much more expensive for my TB..... I guess they know which horses are tough and which are the wooses