Well, a mate suggested using a green scotchbrite pad to scrub the whitewall.
I used one that I regularly dipped in a bucket of wash & wax car shampoo and gave the whitwalls a good scrubbing.
And wouldn't ya know - it worked!
About six weeks or more have gone by since I tried this trick and the whitewalls are still white!
A Thunderbirder from the Land of the Long White Cloud.