She had won the English women’s open match play championship at Kirby Muxloe at the start of the month and was in a four-way lead at the top of the English women’s amateur championship with two holes to play.
But Chelmsford’s Georgina Blackman scored a decisive birdie at 17, a hole which Slater bogeyed to finish fourth behind joint runners-up Sophie Lamb (Clitheroe) and Annabell Fuller (Roehampton).
Three birdies in five holes from the third in Slater’s final round were followed by bogeys at eight and nine, but she remained in strong contention, just one back of the lead on six under.
She stayed in that position after offsetting a bogey at 10 with a birdie at 12. Three pars followed before another birdie, at 16, and she was among the logjam at the top only to be denied right at the finish.
Huddersfield’s Charlotte Heath finished three shots back of Slater in a tie for seventh with scores of 70 70 76 69.
Slater had gone into round two in the top 10 after an opening 71 that saw her accumulate five birdies.
She then added a 69 as four birdies helped her turn in two under. She had got to six under in the round after eagling 13 and following it with a birdie at 14, but bogeys at 15 and 18 denied her a share of the halfway lead alongside Carlyon Bay’s Emily Toy.
After reaching the turn in level par in round three – two bogeys assuaged by two birdies – she dropped a shot at 14, but then birdied 15 and 17.
Heath, who recently successfully defended her Pleasington Putter title, had fired a two-under-par 70 to sit eighth after round one, three shots behind then leader Chloe Howard (Boringdon Park).
Heath laid the foundations for her score with a three-under-par outward nine that included a birdie and an eagle.
The Huddersfield player – who has also triumphed in the Sir Henry Cooper Junior Masters and the English Schools Golf Association girls’ championship this summer – added a second successive score of 70 in round two.
She reached the turn in one under with a couple of birdies and dipped to five under for the event after a birdie at 13. Bogeys at 14 and 15 were partially offset by her gain at the last.
Heath was level par out in the third round, with two birdies, but the back nine began with a double bogey and the single bogeys that followed at 12 and 15 led to a 76.
However, she appeared to have put the torrid back nine pre-lunch rapidly behind her and came out firing in the final round, three birdies in a five-hole spell helping her back into the top 10.
She then broke a run of 10 consecutive pars with a birdie a the last.