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@Spiderweb and
@vickster I'd say the widest tyre the SKS 35s would fit satisfactorily are 28s. If you tried to fit them over 35s, you'd have to mount them further way than optimal and even more detritus would escape left and right. I stayed remarkably clean on Saturday for 70 miles, half of it through muddy Devon lanes (with SKS Chromoplastic 35s over 28s).
By good fortune I had a small flood to cross before emerging onto the final main road. Cleaned my wheels (using 'cleaned' in a comparative sense) very nicely.