Proof if ever it was needed that there are people who know what they are talking about and those that don't. I think Leon must be CAS and i claim my £5.
As a clue Leon sonobuoy usage rate varies depending upon the target, it's speed, it's changes in direction, it's changes in depth, it's noise signature to name but a few. And any! change in just one of these will likely require a relay of a tracking pattern
And of course previous comments about the size of a transit van relates not to one buoy but the several hundred you have to carry to ensure sufficient to track a submarine. And then of course there is weight, but lets not allow that to get in the way of a good theory. A standard A passive directional size buoy, as shown in Biggus's link, are 3 feet long and weigh approximately 31 lbs x a half load of about 100 buoys (note, not a war load) = 3100lbs. Pred B payload is..... Oops! You certainly won't be carrying hellfires and sonobuoys.
Of course the Pred might be doing Indian Ocean surveillance, but again that is ABOVE water not UNDER water. Most on here aren't saying that it can't be used for above water radar picture (assuming of course you can keep clear of cloud and icing conditions) just that there is a lot more to tracking a submarine than people assume. And let's face it, submarines are only going to get quieter! Against some current vessels you wouldn't need your 14 hour endurance, you would have run out of sonobuoys long before that!