London Colney clinched promotion from Herts League Division Six A with victory over third-placed Welwyn Garden City thirds.
Put into bat, with several regulars missing, they saw makeshift openers Andy Cole and Mark Compton score 40 in the first 10 overs.
Seymour (21) and captain Hadley (45) helped lift the score to 195-2 from 41 overs, before Cole fell two runs short of a century.
Colney closed on 244-9 and WGC lost wickets at regular intervals in reply to find themselves 102-5, with Cole (4-42) the chief destroyer.
Simon Fretter dismissed Cope (40) and Uzman Zaman took the last two wickets to seal a 30-run win and take Colney top ahead of a title decider with Old Albanians this weekend.
The understrength seconds had a bad day at the office, although Phil Smith took 4-43 as North Mymms thirds made 172-9.
Colney collapsed to just 51 in reply but hope to bounce back this weekend.
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