GC is 14

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Across
  1. 3. Theo has 2 here George and Liam have one here
  2. 4. George and Liam have one and Theo has 2 although spelt differently
  3. 6. the birthday boy is
  4. 8. there is 3 here
  5. 9. there are 2 here
  6. 10. George is today
  7. 12. 5 are here
  8. 15. it's part of the song we will sing later
Down
  1. 1. 5 people here have one here
  2. 2. we are here to celebrate one
  3. 3. you can't have a birthday without one
  4. 5. there are hopefully 2 here
  5. 7. a game which the party is based on
  6. 11. George and Liam have one Theo has none
  7. 13. you can't have a birthday without one
  8. 14. there are 4 here