Chichester suffered a setback to their London one south promotion hopes when Cobham won deservedly - by 24-16 - for the first time in their five visits to Oaklands.
Their heroic defence was the best seen this season after they had seized on bad mistakes to go 14-0 up in the first 12 minutes.
Chi were always playing catch-up and despite their much greater possession and territory they lacked individual flair and tactical awareness.
Their disappointment was made worse when experienced prop Phil Veltom went to hospital with a broken arm. He is out for the rest of the season.
Cobham are in great form, having also completed a double over Wimbledon and won at Chobham. The final tally was three converted tries and a penalty to two tries and two penalties. The only bright news for Chi was that Gravesend - hot on their heels with a game in hand - had also lost at home, leaving the Blues still top.
The Blues had to change to their red-and-light blue strip. They had made several changes and welcomed back scrum-half Billy Toone from injury.
Chi were in trouble at Cobham’s kick-off, the ball passing over heads, allowed to bounce and then handled into touch. Lineouts and controlled phases were the result before flanker Webb found a gap to score just left of the posts. It was converted by fly-half Farrelly.
Worse followed when Cobham kicked long to the right corner and Chi were caught napping. Only Henry Anscombe got to the ball and, isolated, he knocked on.
Lineout and confident rucks followed with winger Rooke rounding on the outside to score, Farrelly slotting the conversion.
Chi had a mountain to climb but they rose to the challenge and with driving maul and inter-passing they won a penalty 25 metres out which was put away by Anscombe.
Ben Polhill carried strongly and a turnover led to penalties to touch and lineouts in the red zone. The visitors lost a scrum and kicked the ball out in the dead-ball area.
Chi pounded the try line and three times the forwards were held up by determined tacklers. A backs move was also stopped but Cobham were offside and Anscombe’s penalty was easily kicked from in front.
Veltom’s injury occurred as Chi drove again to within ten metres. Polhill went from the back row to hook with Belcher moving to prop.
A good move by Moses Kasujja and Sam Renwick set up huge pressure from mauls and rucks which Cobham somehow repulsed. They did infringe and were penalised several times but the referee kept his pocket closed until after the break.
The visitors were dangerous with a counter-attack and more chances for Chi were spoiled by a crooked lineout throw and then a knock-on in the air. A penalty drifted just right of the posts. It was 14-6 at half-time and game on.
The Blues restarted well with an excellent move across the backs, flanker Chris Johnson driving on and giving a reverse-flick pass as he was tackled.
Cobham deliberately palmed the ball to touch and the penalty put Chi into Vulture’s Corner to continue to batter a brick wall. Cobham manned the barricades and put their bodies on the line.
Within five minutes they were given three yellow cards as the referee lost patience with pulling down the maul, coming in from the side and going offside.
A penalty try seemed justified but was not given. Johnson stormed through two tackles but was grounded by a third.
Chi tried frantically to score while Cobham were depleted but failed.
Left winger Nat Jackson was at last given a run and showed what might have been. Restored to full strength, Cobham’s scrum reasserted their superiorty and Chi could not get quick ball.
The Surrey warriors exerted some pressure at the other end before Chi breached their defence for the first time, Jon Lindsay crossing from a drive maul. The conversion was just missed so it was 14-11.
Cobham then delivered the killer blow. After phases both sides of the pitch they were blocked so went back to the left wing. Right winger Rooke ran across to join in, received the ball on the blindside and found space to squeeze past, accelerate and go around behind the posts. The conversion by the reliable Farrelly meant game over. Chi managed a deserved consolation try for all their toil, the backs running swiftly and Tom Jackson passing to Will Turner to plunge over at the left corner.
Farrelly kicked a penalty for Cobham just before the final whistle.
Chi were left to ponder what went wrong. Polhill had done all he could and had been their best player.
Next, a week on Saturday, Chichester go to Haywards Heath (2.30pm) - all support welcome.
CHICHESTER: Turner, Belcher, Veltom, Blount, Dickin, Johnson, Polhill, Kasujja, Toone, Anscombe, N Jackson, Renwick, T Jackson, Tanner, Cook, Lindsay, Latu, Deed.