I assumed it was just a faulty one, didn't worry about it and put in the other one. Following day, about 12 hours after putting it in, not riding it at all, BANG! The replacement pops too! Same type of tear, different area, an outwards tear in the tube wall.
Is there any point contacting Michelin? I doubt it, probably get no-where and the shop I bought them from (Geoff Smiths Cycle in Bolton) wouldn't replace them, and can't blame them to be honest. Back to Decathlon and eBay for all my cycling needs.