you dont want to connect the washing machine hose to the upstand with a jubilee clip as it needs to be vented to empty properly, one thing to try which is a bit of a pain but could work, disconnect the waste pipe at the copper pipe again, turn off water at washing machine valve, disconnect the cold hose from the back of the washing machine push down waste pipe and turn on,( unless you have a spare hose, if so leave connected to back of machine and fit spare hose to washing machine valve) be ready to turn off if any back flow, but this mains flow should give a good indication if pipe is clear or not