Winchester had a strong first half and deservedly led 2-1 at half-time through Andrew White and Liam Robinson after the home side had had ex-Basingstoke defender Ricky Allaway sent off for a professional foul on White.
But Didcot drew level through Mark Jones just after the hour and the game-changing moment arrived in the 70th minute when Casey saw red for bringing down Pablo Haysham at the expense of a penalty.
With no goalkeeper on the bench, City put ex-Eastleigh defender Ben Wilson between the sticks and the youngster’s first task was to pick the ball out of the net as Elliott Osbourne-Ricketts scored his second of the game from the penalty spot.
Winchester regrouped with three at the back, but the system didn’t suit them.
Morgan Williams capitalised with a couple of stunning late strikes to power the Railwaymen to victory and avenge Diddy’s 4-0 defeat at the Denplan City Ground.
“I’ve never seen such a disparity between a first and second half,” admitted disappointed Winchester owner Paul McCarthy.
“We were superb in the first half. Andrew White had an awesome game and it looked as if we might score six or seven.
“But Didcot came back into it and then Ross Casey has got himself a ridiculous sending-off. He had no reason to even go for the ball. We lost our ’keeper, changed our shape and they ran us ragged.
“Ross is a young lad and I don’t think he’s ready for this level yet, but we’re struggling in the goalkeeping department.
“Rory Anderson is still out with a back injury and I think Jon Webb might have left us to sign for AFC Totton reserves.”
The result leaves Winchester 20th, just four points clear of the sole relegation spot. Worryingly, they have played at least five games more than those around them.