I took them out to Wheelcraft to see what Al reckoned. He cut slightly longer (and better quality) spokes for me and let me build them in his workshop - worked out great! My friend has the axle at the moment to mill the keyway on so can't try them on it yet.
Super chuffed that Al was so friendly and helpful. Anyone in Glasgow area or passing through Scotland, it's worth making the pilgrimage to his shop in Clachan of Campsie, even just for a cup of tea. He recommended Sapim Strong tandem spokes with 13g butting, which I'm really happy with as this is now a super strong wheel that will definitely hold up to the weight
Now the question is what do I do with my extra 72 238mm spokes...