Len,
I have come across a couple of cream cotton suits that had previously been 'hand-cleaned' that had developed a distinct yellowing.
Though the discolouration looked too even to be classic cellulostic browning one responded to B175, mixed 1:7, with a 20 minute dwell.
The next did not respond to the same treatment, though similar in appearance. It was greatly improved by a shampoo with Oxibrite and Fibrebuff, followed by an extraction rinse with B109 the following day.
Another product that comes to mind is Haitian Cotton Shampoo, though I have not used it in these circumstances.