Royal Navy bomb squad recovers torpedo

The Royal Navy is currently conducting an investigation into which submarine/Swordfish is 67 years overdue.
I wasn't aware that the Navy had live-fire "ranges" in the Channel. Or would it have been washed overboard from a surface vessel? Can/will its source be traced?
God know how many torpedos were never recovered from exercises Not to mention the live ones that missed!. IIR a reward was pad for spotting one or recovery. Love to have seen the faces on Vernons gate guard as a cart turned up withan 8 strapped on the back. Btw What was a mk9? And do we still have any mk8's in service?