If you're an English melodeon player who's serious about being 'traditional', you almost certainly (indirectly) picked it up from someone who heard Walter Bulwer's recording on Reg Hall's classic recording of the Bulwers of Shipdham play the third part in G, like he did.
If you're from any other tradition, follow that but I'd point out that Whitby is in England & English players are entitled to do it any way they want in their own country.