the St Merryn Brentons moved there from St Wenn in the first part of the 18th century. Philip Brenton and jane Knight had Henry b.1758,William,b.1755, Jane, b.1760 and Prudence b.1763 in St Merryn.
Henry and Frances Old had Henry b.1789, John b 1797 and Richard, b.1800 (my 3x gt grandfather)who moved to Padstow. Henry 1789 had a son called Henry b 1828 in St Merryn and maybe he had a son called Henry. I would guess your Harry name might come from Henry, but without knowing your grandfather's name, that's all I can say.
Maria Brenton (London)