My low-level cistern has a button to press to flush the loo. If I
simply press the button I get water hammer noise. If I press it
initially very gently, so as to release some water into the bowl, and
then press fully down, I DON'T get water hammer.

So where is the *actual* water hammer noise emanating from? I've read
before that water hammer is something to do with high pressure and
valves, but I don't get how this only started recently (over the past
couple of months).

This is the cistern internal 'gubbins':
http://www.littletyke.myzen.co.uk/cistern/index.html

MM