Wimbledon deserved their 33-22 home win over Chichester in London one south and became the only team this season to do the double over Blues, having won 24-22 at Oaklands.
It was no surprise as Chi were without five key players from their great performance against Gravesend. They were disjointed and their weakness was exposed by some excellent Dons counter attacks with speed and slick handling to the wings.
Their scrum was good enough but the lineout misfired at times and mutual support between forwards and backs was lacking in the open play.
Their speed merchant on the right wing was starved and the centres were stopped by positive tackles. However, Chi scored four tries to earn a bonus point and guarantee a promotion play-off against either Eton Manor or Westcliff from London one north.
Whether they will be home or away depends on results from final games this week, but they will have to improve significantly if they are to be successful on April 26.
Their outside chance of automatic promotion disappeared with Gravesend’s 41-15 defeat of Brighton.
Will Turner, Joe Woods, Niall Goldie, Richie Adams and Sam Renwick were unavailable or injured. Dane Gingell came in at hooker, Nick Shopland at loose head prop, Chris Johnson at No8, Jon Lindsay at lock and Billy Toone at scrum-half with Henry Anscombe moving to fly-half.
A couple of early mistakes and an injury to Toone were not a good sign. They attacked to the left 15 metres out but lost a lineout and scrum.
Anscombe kicked intelligently into space from halfway and a helpful bounce allowed full-back Toby Golds to gather and pass outside to left winger Ben Robson. He dived over for the first try, the conversion held up by the wind and falling just short.
Dons countered to the right, recycling to the middle where Hallett found a gap left when Golds lost his footing and scored, converting his own try.
Blues went to the left corner again with a penalty, lineout and driving mauls, but when the ball was spun out Dons cleared.
Hallett unusually missed a penalty before Rory Tanner had his only run in the game and Toone was tackled into touch.
Dons probed with a scrum and tap penalty and Chi defended but after winning a scrum against the head they promptly gave the ball back and Hallett kicked a penalty from the resulting ruck.
Johnson powered forward and with a penalty, lineout catch by Nick Blount and confident drive, the pack rolled across for Gingell to touch down for an unconverted try.
Dons went straight back and with quick ball from their scrum, their passing forced a break. A Chi defender drifted when he should have tackled and Golds faced a two on one situation. A firm final pass put right winger McDonald over at the right corner, though the conversion was missed.
Another mistake when the ball was short-kicked over the top on the Chi 22 straight to a Don led to a penalty at a defended ruck. Hallett obliged with three points to leave the Blues 18-10 down at the break.
Chi were relying too much on their forwards as the centres had been receiving man and ball together, the referee not concerned about offside at the tight.
Johnson made a good catch and Dons infringement against a maul gave a yellow card to to a lock forward. Chi knocked on at a lineout but more pressure saw the pack held up with Gingell inches short.
Dons defended resolutely but had another yellow card, against the open-side flanker. More five-metre scrums followed and Anscombe sidestepped neatly to score near the posts and it was converted.
Dons hit back and after Jack Bentall had stopped his man with a crunching tackle they won a penalty. A tap penalty by the fly-half and long accurate passes with good support gave May an unconverted try at the right corner.
Robson had been yellow carded for a high tackle and Dons took full advantage with a flowing move, full-back Moore entering the line to finish off their fourth try. Hallett converted.
Chi were dispirited and the Dons scrum-half was allowed to run from behind his posts to the halfway line without being tackled. Hallett kicked his third penalty in front of the posts.
Chi concentrated on scoring a fourth try for a bonus point. The forwards hammered at the door, Ben Polhill and Moses Kasujja held up just short. Another Don was sinbinned as they stubbornly refused to give way.
Kasujja had another lunge for the line and then Anscombe switched the ball to prop Tom Belcher, who scored his 14th try and Chi’s 124th of the season at the right corner flag.
This week, the Blues go to Brighton (3pm). All support welcome.
CHICHESTER: Shopland, Gingell, Belcher, Blount, Lindsay, Dickin, Polhill, Johnson, Toone, Anscombe, Robson, Bentall, Cook, Tanner, Golds, Davies, Kasujja, Driscoll.