Wm. Sheen (entered 1755) looks to be too early and Wm. Sumner (entered 1802) too late, so that leaves perhaps Wm. Sutton, entered 1784. Any confirmations/denials?
The mark is perhaps that of William Sharp see viewtopic.php?f=74&t=23602 Although primarily a buckle and tong maker I suspect he may well have bought in spoons. The mark certain looks like several I have on buckles attributed (by me !) to Sharp.
Am I missing something here (probably), but is the mark not clearly that of William Summer I (Grim. 3319)? He's the only spoonmaker amongst the names mentioned working at that date.
I am with Trev on this one, I think it is also Wm Sumner I as I purchased a basting spoon by Wm Sumner I last week, date 1794 with the exact same makers mark as the one in the OP's post.