Hi Neil, this is a favorite subject for me as it develops so much business for us, as you know i have a total aversion to the use of high alkaline based cleaners, we come across so many facilities where the white grout lines have been chemically burned from the continuous use of high alkaline based cleaners which turns them yellowish, the only solution to this problem is to use a grout colorant, we always recommend Steve from groutperfect.co.uk for our clients as he is very knowledgeable in this area, our clients always come back to us with positive comments in dealing with groutperfect, then we can recommend safe cleaners for their tiles and grout which will not cause detrimental damage from continuous use, hope you find this information of benefit.