I don't have canvas spectators, but for white canvas plimsolls, I spray them with a 50/50 mix of water and lemon juice and gently scrubbed them for a bit with a stiff toothbrush. Once they're dry, they're a bit brighter. For tougher stains, I mix a tablespoon of baking soda with a touch of water and rub the sticky paste onto the stain (again, with a toothbrush). Let the paste sit for a bit, then brush it off, and usually a bit more of the stain will go away.
I'm hesitant to really recommend this with your Malverns, however. It might be good to try it out on a cheap pair of white plimsolls, or some canvas shoe you don't care about, and see how they fare. After that, try it on an inconspicuous place on the Malverns. I do it without hesitation to my canvas shoes because they only cost something like $50.