Movies
The comedy, which stars Jason Segal, Kristen Bell and Russell Brand, took in an impressive £2 million haul, leading nearest rival The Eye by some distance - the Jessica Alba horror film grossed just over £700,000.
After two weeks at number one, card counting film 21 slips down to number three.
The Hugh Jackman thriller Deception was the only other new chart entry, coming in at number seven.
The top ten in full:
1. (-) Forgetting Sarah Marshall - £2,144,038
2. (-) The Eye - £704,655
3. (1) 21 - £547,866
4. (2) Fool's Gold - £476,244
5. (3) In Bruges - £408,109
6. (5) Son of Rambow - £278,529
7. (-) Deception - £255,958
8. (4) Street Kings - £250,634
9. (6) The Spiderwick Chronicles - £238,362
10. (9) Happy-Go-Lucky - £238,099
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