Can anyone tell me if there is a tee peice on the market which will allow me to connect a 35mm waste pipe for a washing machine drain into a 63mm down pipe?
No there isn't because of the mixing thing - you shouldn't "need" to use something llike that. You can go via 110 soil pipe size, or you can use a rubber connector. There are two which would do it, one is a step-down rubber tube with Jubilee clip on each end, and the other is a "bush" which will go into a 63mm boss and seal onto 40mm waste.
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