Just for anyone else who may be facing the same issue. Our ADSL2+ connection has been badly playing up, probably since early February.
Everything I had tried earlier had no positive impact. Finally what seems to have resolved it is putting the ADSL modem/gateway into 'Bridged Mode' and handing over all the smarts to our ASUS wi-fi router (putting PPoE credentials into the router etc).
I also added custom DNS server settings into the router which earlier had no positive impact. Not certain how critical this is with the modem/gateway operating in Bridged Mode as the issue appeared to be resolved even without this (though I could be wrong, as this whole issue appears to be DNS related...)