My query over 30m of 20mm water pipe and a solenoid valve with regards to water hammer has been found not to exist.
It seems that the solenoid valve I have isn't "direct drive" so it opens and closes with a slight delay. I have however stuck up a youtube vid showing the horrendous racket given off by the cleaning pump (which I suspect might be a faulty unit to make such noise) and the silence and efficiency of having replaced it with a solenoid valve.
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The additional "T" into the pipe work is capped off with another solenoid valve which might get used either as an auto top-up for the pond or I might try and rig up an out-door IR sensor hooked up to a water sprinkler (something like
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" to keep the cats away...
:)
And a camera to catch the action ;)