Ian Chalk’s men stamped their high-flying authority on their neighbours within five minutes as Tyler Brockway finished Jack Eves’s cross.
Kane O’Keefe broke the offside trap minutes later to double the Western Way outfit’s lead, and a simple third from Joe Sanger, following a flamboyant move with Jack Slade, put them out of sight.
Declan McGregor hit a mere consolation for the Robins before they slumped to a 27th league defeat.
“We went out to hit three past them early on and the game was over in 30 minutes,” Chalk said.
“All three goals were wellworked and I don’t think Slade and Sanger realise how good they actually are. They certainly have the talent to go further.
“We were exceptional in the first half and played with lots of freedom and confidence.
“Having lost just two games in 12 is fantastic. It was a job well done.”
“Now we end our campaign at Folland Sports on Saturday (3pm) and we’ll give it a good go.”
Meanwhile, Downton, now dragged back into the bottom two, have to win their final clash at home to Blackfield & Langley, and hope Fareham win at Fawley, to avoid relegation.
And they will have to do it without Craig Lewis and Jamie Walters, as well as former skipper Matt Palmer, who stormed off at half time during the derby clash.
“We were terrible,” admitted boss Jeff Softley.
“Without taking anything away from Harlequins, we were as bad as we’ve been all season.
“I’ve said to the boys I don’t care if we lose, as long as we play good football.
“We just looked a bit worried and it was disgraceful that Matt left at the interval.”
“I still believe we can stay up and I’m that confident Fareham will beat Fawley, I’ve told my players if they win on Saturday they will be playing Wessex Premier football next season.”
Softley’s counterpart Chalk is also confident Downton will get themselves out of the dogfight.
He added: “I think they will get a result at home on Saturday and stay up.”
Find out if Downton survive in next week’s Journal.