The range country of Lion-tailed macaques is India, a fairly wealthy (though unequally so) country, with a history of colonial domination by Britain. If you research some of the struggles around silent valley, you will see there have been significant battles to conserve this species in situ that haven't required a european zoo to step in and coordinate efforts.
I could have missed something, indeed if Chester or any other zoo are contributing funds to in situ work with this species, it would not necessarily be listed in the mammal section of their annual report - most collections have a specific section detailing their in situ work further on in the annual report.
Therefore, as with other European collections holding this species, you might find that Chester just hold a managed group as part of the EEP. I would look at the EAZA website/activities/collection planning for further information about who coordinates the EEP for lion-tailed macaques currently.