A final question, if I were to remove the current washing tap at the top and replace it with a check valve would I be able to use the same olive and nut from the short pipe just below it, as long as I put some PTFE tape on the old olive?
Looks like it has a raft of ptfe on it , a sign that it may have been problematic in the past. As long as the nut is the same thread as the valve ,you could try it. If it weeps ,use your pipe slice ( someone recommended you buy one in an earlier post ,and so do I ) cut pipe just below olive and fit the new olive and nut that came with the check valve. By the way ,if you decide to cut the plastic pipe on the hot supply side ,be aware that plastic pipe must have a pipe stiffener inserted into the pipe ,before attaching a fitting.
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