Dear Lorna,
Unfortunately your ggg grandfather died before my records start, but as these professions run in families I did look for other Fletcher's and found:
Charles Fletcher born 1854 in Strath, Invernesshire Rank: Mate Vessel: Mary List date: 1/1/1894
Duncan Fletcher born 1875 Invernesssire Rank: Ordinary Seaman Vessel: William Berey List date: 1/1/1892
I know these are too late for you, but do you think that there might be a connection with Strath for your Fletcher's?
Alternatively, I can go to Kew and look through the actual records for John as these give ages and places of birth. There are two sets of records:
Register of Seamen Series II, 1825-1844
Register of Seamen's Tickets, 1845-1854
Sadly, this is a paying service so if you're interested send me a personal message or an e-mail.
Best Wishes, Michelle