Try soaking the filet in some milk for a 2-3 hours. This helps break down the piss and tenderizes the flesh a bit.
It's what we used to do for Mako when served in some of the restaurants I worked in.
Also, I think the Brits use a different type of dogfish, not sure though.
BN
I had a box of animal crackers the other day. The box said 'do not eat if seal is broken.' I opened the box, the lion was OK, the elephant was OK, sure enough, the darn seal was broken.