An update. I got possession of my Boswell today and showed a gunsmith the off centre trigger guard tang. He was very reluctant to adjust it in any way and commented that it might have been made this way by the makers. As Redoak has posted, he has also noticed this on some of his English guns. Although it looked a little odd when I first noticed it, I've decided to leave as is and enjoy the gun shooting clay targets at my home club.