It may be some comfort to know he’s not the only MPK casualty from HMS Campbeltown or the MLs that accompanied her on the raid. I’ve not checked them all but out of the 111 RN & RM casualties who died on 28 March 1942, I’ve seen at least 10 others who are commemorated on either Plymouth or Portsmouth Memorials and who have the ship (not an ML) recorded as their unit.
You can access the list by doing an advanced search on the CWGC’s web site, leaving names blank, just fill in the date and Royal Navy. Also if you didn’t see it, you can read the historical information about the Memorial by clicking on either “Plymouth Naval Memorial” or “Find out more” in the section just under his details.
Although it will not give much detail, you could obtain a copy of his death certificate online from the Overseas records of the GRO ; the reference to order it is included in the GRO Index of War Deaths (Naval Ratings) 1939-1945 and is:
Name: FINDLEY, William R; Ship or unit: Campbeltown; Rank:Able Seaman; Year: 1942; Vol : 4 page 1286.
GRO certificate ordering online is at http://www.gro.gov.uk/gro/content/certificates/default.asp
or you can email them or give them a call - click “contact us” from list on right for details.