HAPPY Valley gatecrashed Eastern's post-Christmas party by pulling off the upset of the year in a stunning 3-2 First Division victory last night. Happy Valley were written off before the game following their disastrous 4-0 defeat by Sing Tao a fortnight ago, but they found their form when it mattered, handing runaway leaders and reigning champions Eastern their first league defeat of the season and only their third domestic loss of the year. Eastern's only other domestic losses this year were in the final of last season's Viceroy Cup where they lost 3-0 to South China, and a 2-1 league reverse against Sing Tao, also last season. Before last night Eastern had won all seven of their league games this campaign. Import Ricky Cheng Siu-chung, making his debut, was Happy Valley's new-found hero. He fired home the winner midway through the second half and throughout showed the form which won him a regular place in the Costa Rican under-19 youth squad two years ago. The final whistle sparked off wild celebrations by Happy Valley after they had endured a tense four minutes of injury time. Cheng was mobbed by the crowd who spilled on to the Mongkok Stadium pitch. The victory moves Happy Valley to fifth place, one point behind Instant-Dict. Cheng, the 21-year-old son of former Hong Kong international Cheng Kwok-kun, could not contain his delight, saying: ''I was a bit apprehensive at first because I didn't think I was very fit. The style here is a lot different to Costa Rica so it's really pleasing to have won the match against as strong a team as Eastern.'' Happy Valley's normally staid team manager Leung Chi-ming was grinning from ear to ear after the win which blemished Eastern's otherwise perfect record. ''All the players played well and we fully deserved the victory. Cheng played a great game and we should see a lot of him once he becomes match fit. He's not accustomed to Hong Kong soccer so he should play even better as the season progresses.'' Left-back Lo Kai-wah, who is nursing a knee injury and goes under the knife today, was a conspicuous absentee at the heart of Eastern's defence. Eastern experimented by first putting Chiu Chun-ming and then Lee Wai-man in Lo's role. Eastern coach Chan Hung-ping was stunned following his team's first defeat saying: ''We can't really put our defeat down to the fact Lo was not playing. Happy Valley are a strong side and that's a fact.'' Eastern looked headed for their eighth straight league victory when Tony Sealy rose high above Happy Valley's defence and headed in Tam Siu-wai's 13th-minute corner. But Happy Valley equalised when Wei Kexing met Keung Wing-yiu's cross in the 18th minute. Ross Greer gave Eastern a 2-1 lead going into the interval when his powerful left-footed shot went in-off the cross-bar. But Happy Valley were still full of fight and were rewarded with the vital two goals in the space of 12 minutes. First Chau Chiu-hung crashed home the equaliser in the 58th minute. Then Cheng made his decisive strike, beating 'keeper Iain Hesford from 20 yards after Tim O'Shea failed to clear a Happy Valley free kick.