Born and bred in Middlesex, I went up to Edinburgh to Uni and stayed in Scotland for 12 years, the last seven in Glasgow. As a teacher I tried to adapt my ghastly English accent so the pupils could understand words for eg spelling tests. It's no good saying 'girl', 'pearl' and 'curl' with exactly the same vowel sound. In Scotland it's 'girrel', 'perril' and 'currel'. I've quite a good ear for languages and accents. In singing lessons we teachers were given many old Scots songs to teach. I'd have sounded absolutely daft if I'd stubbornly delivered eg Will Ye No Come Back Again? in broad English. As for Come to the Barrowlands Tonight, (Swing yer ma, swing yer pa, swing yer grannie through the wa' etc ) just imagine that in 'posh West Londonspeak'! I also had to coach some pupils in Robert Burns poems for a competition. Luckily a Scots colleague was at hand to help with the Burns words and pronunciation. So I do feel the song determines the accent to a great extent.