I know that if I know the direct link I can access the file. 

e.g. <http://ais.channel4.com/subtitles/3657676>

But if I want to download all the files with just the directory structure, it
doesn't work and I get the forbidden 403 error. 

1. I have tried with robots.txt on and off
2. Tried nearly all the Browser IDs with no success

HTTrack3.33+swf launched on Sat, 12 Dec 2015 12:24:37 at
<http://ais.channel4.com/> +*.png +*.gif +*.jpg +*.css +*.js
-ad.doubleclick.net/* +*.smi +*http://ais.channel4.com/subtitles/*
(winhttrack -qYC2%Pns2u1%s%uN0%I0p3DaK0H0%kf2A25000%f0#f -F "Mozilla/4.0
(compatible; MSIE 6.0; Windows NT 5.0)" -%F  -%l "en, en, *" -Y
<http://ais.channel4.com/> -O "C:\My Web Sites\All4,C:\My Web Sites\All4" +*.png
+*.gif +*.jpg +*.css +*.js -ad.doubleclick.net/* +*.smi
+*http://ais.channel4.com/subtitles/* -%A
php3,php,php2,asp,jsp,pl,cfm,nsf=text/html )
Information, Warnings and Errors reported for this mirror:
note: the hts-log.txt file, and hts-cache folder, may contain sensitive
information,
 such as username/password authentication for websites mirrored in this
project
 do not share these files/folders if you want these information to remain
private
12:24:37 Info:  Note: ais.channel4.com robots.txt rules are too restrictive,
ignoring /
12:24:37 Error:  "Forbidden" (403) at link ais.channel4.com/ (from
primary/primary)
12:24:37 Info:  No data seems to have been transfered during this session! :
restoring previous one!