Well that didn't last long, complete noob mistake.
Put the cory's in a small quarantine tank that hadn't been cycled properly, just seeded with the filter media from my main tank.
Three days later, a nitrite level of 1, and three dead cory's. To make it worse i was away with work, i might have done water changes to keep the water safe if i was at home, but got the call from my wife to say they had expired. I am gutted but it's my own fault for not taking my time and doing things properly.