Thanks Mehdi,
The list renders with the Masonry template, but it seems the changes to Items in the list can only be made in seb_minima for my config.
It seems I can only set Masonry for the List, but have no option to set it for Item. Therefore overriding in seb_masonry/positions/list_xyz/item/mainbody.php has no effect, whilst doing the same in seb_minima/positions/list_xyz/item/mainbody.php does have an effect to the items (to re-emphasise, I do not have the drop down option in the list config to set the Item template to seb_masonry, only seb_one and seb_minima)).
Thanks!