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Tonbridge Angels are 90 minutes from promotion after winning the Bostik Premier play-off final on Monday.
They beat Merstham in front of more than 2,000 at Longmead with Chinedu McKenzie and Joe Turner scoring the crucial goals.
Now the Angels travel to Met Police on Saturday for the 'super play-off' to decide who will play in National League South next season.
Tonbridge were unchanged from their semi-final win at Haringey although Joe Healy was drafted onto the bench, with goalkeeper Deren Ibrahim also named among the substitutes.
Kick-off was delayed for 15 minutes with so many people queueing to get into the ground and Angels made an assured start in front of the bumper crowd.
Turner's volley deflected wide from 20 yards and when D'Sean Theobalds delivered the corner from the right, Sonny Miles claimed he was being wrestled in the middle only for referee Michael Ryan to rule the other way.
Tonbridge were slick in their passing and McKenzie offered a good outlet down the middle, although Fabio Saraiva went close for Merstham when he cut in from the left and fired a low shot wide of the near post.
But most of the early play was in the visitors' half and McKenzie had a shot saved by Amadou Tangara, the close attention of two defenders doing just enough to put him off at the vital moment.
Merstham were set up to strike on the counter-attack, with the pace of Walter Figueira and his colleagues their chief weapon. Figueira got away from James Folkes too easily down the left but whipped a shot into the side netting when he should have got his head up and picked out a team-mate.
Kershaney Samuels created an opening for Saraiva and it needed a good challenge from Miles to block his shot as the Moatsiders started to get on top.
But just as the home crowd had started to fall quiet, their side hit the front on 34 minutes. Joe Turner fizzed a low cross towards Tom Derry at the near post and although Merstham made the first block, the ball ran kindly for McKenzie near the penalty spot and he rammed it home.
Tonbridge were heading serenely towards the interval when a big flare-up saw both teams reduced to 10 men. Sonny Miles and Oliver Cook clashed following an aerial challenge in the Angels box and Mr Ryan sent both players off, leading to a tactical reshuffle by Tonbridge boss Steve McKimm.
Craig Stone filled in at centre-half alongside Arthur Lee, with Turner moving inside from the wing and McKenzie making up the midfield, leaving Derry as the lone striker.
Some patient build-up by Merstham at the start of the second half created a shooting opportunity for Figueira but he was all of 30 yards out and couldn't generate the power necessary to worry goalkeeper Jon Henly.
The visitors were getting a stranglehold now, with Angels dropping deeper and deeper as the pressure began to mount.
Steve McKimm responded by sending on Healy for Derry, allowing Turner and McKenzie to resume their regular positions and the move helped Angels starve the visitors of the ball for quite some time.
Figueira took up a good position in the Angels box but failef to pick out a team-mate and Tonbridge almost doubled their lead on the break. McKenzie set off down the right, slipped the ball inside to Theobalds and his pass on to Turner was drilled back across Tangara but just wide of the far post.
A good diagonal ball looked to have played in substitute Daniel Bennett but the flag went up for offside and Henly grabbed his attempted lob anyway.
Turner's clever turn got Tonbridge breaking forward and he linked with Alex Read, just on from the bench, but his touch allowed the Moatsiders to clear their lines.
You could sense the frustration in Merstham's play as the Angels defence held firm, captain Simon Cooper smashing a shot well wide from distance as time started to run out.
Henly held Roman Michael-Percil's long shot as the home side kept a good shape and denied the Moatsiders space in the final third.
Jack Parter almost doubled the Angels lead late on, surging into the box as Merstham's defence opened up in front of him, with Tangara racing out to spread himself at the full-back's feet.
Parter was involved in the move which led to Tonbridge's second goal in the 90th minute. He fed Theobalds and when the midfielder beat one man and was fouled by another, Mr Ryan pointed to the spot and Turner smashed his penalty straight down the middle to start the celebrations.
Tonbridge: Henly, Folkes, Parter, Theobalds, Miles, Lee, Ramadan, Stone, Derry (Healy 55mins), McKenzie (Read 73mins, Small 81mins), Turner. Subs not used: Smith, Ibrahim.
Merstham: Tangara, Cooper, Harrison (Abnett 57mins), Hutchins, Cook, Kavanagh (Bennett 65mins), Samuels, Beany, Figueira, Sow, Saraiva (Michael-Percil 75mins), Subs not used: Davies, Henriques.
Attendance: 2,268.