I think Twycross will be essential to this one; without a visit to The World Primate Centre© this year, I'm on a fairly meagre 29 (which goes up to 37 if I cheat by including mainland European zoos).
London Zoo, January 2nd
1. Western lowland gorilla
2. Northern white-cheeked gibbon (+ Wingham)
3. Diana monkey
4. White-naped mangabey
5. Kikuyu colobus (+ Banham)
6. Sulawesi macaque
7. Francois' langur
8. Ring-tailed lemur (+ Banham, Wingham, Hamerton, AA!, Tropiquaria)
9. Northern slender loris
10. Potto
Banham Zoo, February 22nd
11. Black & white ruffed lemur (+ Wingham, AA!)
12. Sclater’s lemur (+ AA!)
13. Northern bamboo lemur (+ RSCC)
14. Red ruffed lemur (+ Wingham)
15. Red bellied lemur (+ AA!)
16. Siamang
RSCC, April 16th
17. Red-cheeked gibbon
Wingham Wildlife Park, April 16th
18. Barbary macaque
19. Grivet
20. Mandrill
21. Mueller’s gibbon (hybrid)
Hamerton Zoo, April 17th
22. Collared lemur
23. Lar gibbon (+ Tropiquaria)
Tropiquaria, April 19th
24. White-fronted lemur
Africa Alive!, May 25th
25. Black lemur
26. Crowned lemur
27. Mongoose lemur
28. King colobus
29. Chimpanzee
Antwerp Zoo (April 7th)
1. Northern slender loris
2. Red ruffed lemur (+ Artis, Avifauna)
3. Ring-tailed lemur (+ Artis, Blijdorp, Avifauna)
4. Sulawesi macaque (+ Artis)
5. Mandrill (+ Artis)
6. Hamadryas baboon
7. Hamlyn’s monkey
8. Javan langur
9. Siamang
10. Common chimpanzee
11. Eastern lowland gorilla
12. Western lowland gorilla (+ Blijdorp, Artis)
Blijdorp (April 8th)
13. Ring-tailed lemur (+ Avifauna)
14. Lion tailed macaque
15. Mantled colobus
16. Francois’ langur
17. White-naped mangabey
Amsterdam Zoo (April 9th)
18. Grey mouse lemur
19. Black & white ruffed lemur
20. White-throated guenon
21. Red-cheeked gibbon
22. Chimpanzee
Vogelpark Avifauna (April 10th)