On Thu, 14 May 2015 10:23:28 +0100, EricP
wrote:

Anything to be done? Is it likely that I'm hearing the sound direct
or through our water pipes? How do I ascertain?

Ask your neighbour if they mind you fitting a low noise filling device
to their attic tank such as
http://www.fluidmasteruk.com/our-products/pro-series/pro-side-entry-fill-valve-with-heavy-duty-brass-shank.html