The Reds' unbeaten run, stretching back to New Year's Day, ended at Castle Park and with it went the dream that Jersey's impressive recent surge would take them into the Greene King IPA Championship play-offs.
After an early exchange of penalties, there were two tries in the first half, with Tyson Lewis crossing for the Knights after a length of the field move, and the Reds replying with a penalty try. Lewis's try, his fourth in six outings against the Reds in the past two seasons, came after a length of the field move that followed an intercepted pass from Jake Woolmore that would almost certainly have seen Nick Scott score for the Reds at the other end.
After another exchange of penalties, a powerful maul set up the visitors' opening try as the game moved into the second quarter. Richard List dragged the maul down and referee Greg Mcdonald despatched the Knights' prop to the sin-bin immediately after signalling for a penalty try.
The home side led 16-13 at the break with Callum Sheedy missing a long-range penalty with the last kick. The Reds' fly-half was also unsuccessful with an effort from closer range early in the second period.
A scrappy second half featured few clear-cut scoring chances, but on 54 minutes, Flockhart crossed for a try after sustained pressure.
The Knights might have had another try shortly afterwards. Alex Shaw stole the ball at the back of a line-out inside the Jersey 22 and broke forward, possession was worked inside then quickly back out with Lewis looking to have scored in the corner, after some deliberating the referee revoked the try concluding that Lewis and Andy Bulumakau had crossed.
Flockhart added a penalty to make it 24-13 as the game entered the last 10 minutes - a handy cushion for the home side.
Eventually the Reds were able to score through former Doncaster prop Jake Armstrong, but it was too little, too late, as the conversion by Brendan Cope - making a welcome return to action after five months out with injury - was the final kick of the game.
With Ealing gaining a big win against London Scottish, and Cornish Pirates denied at the death by Richmond, the Knights and the Trailfinders claimed the last two play-off spots, alongside London Irish and Yorkshire Carnegie, although the semi-final pairings aren't yet known. Though they could match Doncaster's points tally, the Reds cannot overtake the Knights due to winning fewer games. However there is still much to look forward to: the final Championship match against Richmond, in which a win would secure Jersey's highest-ever league placing (fifth); the B&I Cup Final against Munster 'A' on Friday April 21st; and the Siam Cup against Guernsey on Saturday May 6th.
The result maintained the tradition of very close games between the Reds and the Knights - in 10 contests there has never been a winning margin greater than seven points, with four wins apiece and two draws.
Doncaster Knights: Jarvis; Flockhart, Bulumakau, Hayes, Lewis; Humberstone, Heaney; List (YC 21, Bergmanas 56), Hunter (Nelson 67), Quigley (Sproston 63), Challinor (Williams 63), Young, Ram (Bergmanas 28-31, Edgerley 75), Hills (Carpenter 67), Shaw. Sub not used: Cusack
Jersey Reds: Cuthbert; Watkins, Davies, Ma’afu, Scott; Sheedy (Cope 51), Hardy (Dudley 69); Woolmore (McCarthy 57), Buckle (Selway 57), Kerrod (Armstrong 60), Phillips, McKern (Kolo’ofai 47), Freeman (Argyle 64) Graham, Haining.
REFEREE: Greg Macdonald. Assistants: Phil Watters, Ross Campbell
ATTENDANCE: 1,698
Half Time: 16-13
SCORERS
DONCASTER KNIGHTS
Tries: Lewis 10, Flockhart 64
Conversion: Flockhart 10
Penalties: Flockhart 2, 8, 30, 72
JERSEY REDS
Tries: Penalty Try 21, Armstrong 80
Conversions: Sheedy 21, Cope 80
Penalties: Sheedy 4, 18
