I got a response from Cilit Bang today,
Reference No: 14541840
Thank you for your email regarding Cillit Bang
We would not recommend the use of Cillit Bang for preparing steel sheet or tubing before brazing or soldering, as it has not been tested for this use.
I’m sure that’s from their legal department rather than their technical department. So there recommendation is not to use something without testing it first, a sensible suggestion, I’ll have to think up some appropriate tests.
Apart from Aggy and Kim, it would appear that there’s some other cleaning experts out there. The Toxics Use Reduction Institute have a database of cleaning solutions.