Hello, Jan. I've made a bit of a breakthrough using 'The Genealogist' website. Guess who I've found in the St Giles & Bloomsbury workhouse in the 1841 Census:
William Hamlyn, butcher, aged 60, not born in county.
I've found what looks as if it could be his death, too - in the March quarter of 1843, in the St Giles district. I'll send off for it and report back.
This may tie in with your idea about William and Johanna Hamlyn, who had a child named Emma who was christened in St Botolph, Bishopsgate in 1835. IGI also records a marriage between a William Hamlyn and a Johanna Sullivan in 1834 in the Old Church, St Pancras. Of course, the Devon connection may still be there, since he wasn't born in the county, but I've no idea how I'll trace him further back.