Heya Steve
The Baskers have been a labour of love and frustration for me! I've seen them rarely, but when I have it's never been possible to film them for one reason or another. Poor weather/sea conditions, 0m vis, can't get close enough to them etc etc.
I've never heard of anyone seeing them while diving on a reef like you would see a Whaleshark for example, so you really need to actively go out and look for them. They can been seen at the surface for only a few months a year. Occasionally a group of divers will spot some feeding at the surface between dive sites and all jump in to take a look, but that's rare. The rest of the time they spend in deep water below recreational limits (as far as anyone knows).
You can charter a boat for a week of searching for them well in advance, only to find that despite ideal conditions there none are around. They are one of the most unpredictable sharks around, so it's no wonder you haven't seen a lot of footage of them! 
I'm already organised for this year, but if anyone wants to get together in Cornwall or the IoM next year then we could charter a boat and go looking for them. Considering my past success rate though you may not want to take me along!! 
Cheers, Simon