I have been watching this white patch in the corner of the radiator carefully as it is seepage and a sign of a small leak in the radiator. I put some radweld in about a year ago which seemed to cure it. Since having the engine out and refilled the cooling system with just water the white patch had reappeared.
Think will try the radweld solution again.
Yes and after 14 months no sign of a leak – good stuff!