Just answered my own question .... Korosi red is listed with RS Motorcycle Paint, RS Bike Paint, a 400ml aerosol is £22. Why are Triumph so secretive about their bike paints??, Tw**s.
Apparently (from another post on this site), Ral 3020 - Traffic red is a good match for Korosi red, this is available on Amazon for £12.99 for 400ml (Acrylic)
Do you have a body shop supplier nearby? They can mix up a custom spray can for you. Bring an example of the colour you want and they match it. Rondex in Canada here does it. A bit more $ though.