Cool!
Ray and shark skin make amazing leather -- and remember that they were also both used for the wrapped handles of samurai swords too, so you might have a nice wrap for a knife.
I've never tanned fish hide, but from what I know, the key is to get the existing fish oils out. Urine has traditionally been used (just like dog poop has been used to bate cow and deer hides.... tanning can be an olfactory adventure sometimes, although it's better now than in the old days....)
Here is one traditional recipe:
http://wiki.answers.com/Q/How_do_you_tan_fish_skin
Here's another (about midway down the thread):
http://www.ausfish.com.au/vforum/archive/index.php/t-113185.html
Taxidermy supply houses typically sell kits to tan fish skin -- so you might look online to see what you can find in UK taxidermy.
By the way, if anyone has salmon skin, it can be tanned into a nice leather too.
And for really amazing leather: bullfrogs. It's not very big, but it's fabulous stuff when you tan it yourself....
If you can, please post a follow-up with your experiences. I for one would love to read/see it.