OK so we did some testing. We hooked up one computer directly to the PCCW modem via a cable connection; the other two via wifi and waited for 8pm. When the internet cut out on the wifi computers we went and tested the cable-connected one.. and ... it also cut out. It's not a wifi problem.
The PCCW man came and changed our modem yesterday - let's see if that helps.
However, I also talked to one of my neighbours and they are getting exactly the same problem, at exactly the same time of day. Their take was that we have a "bandwidth hogger" in the marina - some software that steals bandwidth. My neighbour thinks one of the kids has probably installed it and doesn't realise the impact. Does anyone know anything about these? Is is a credible idea and if yes is there any way to trace it or block it?