Whilst trying to update, I get the following:
Ign http://apt.osmc.tv jessie InRelease
Get:1 http://mirrordirector.raspbian.org jessie InRelease [12.2 kB]
Ign http://apt.osmc.tv jessie Release.gpg
Ign http://apt.osmc.tv jessie Release
Ign http://apt.osmc.tv jessie/main armhf Packages/DiffIndex
Get:2 http://mirrordirector.raspbian.org jessie/main armhf Packages [9,531 kB]
Ign http://apt.osmc.tv jessie/main Translation-en_GB
Ign http://apt.osmc.tv jessie/main Translation-en
Err http://apt.osmc.tv jessie/main armhf Packages
404 Not Found
Get:3 http://mirrordirector.raspbian.org jessie/contrib armhf Packages [43.2 kB]
Get:4 http://mirrordirector.raspbian.org jessie/non-free armhf Packages [84.2 kB]
Ign http://mirrordirector.raspbian.org jessie/contrib Translation-en_GB
Ign http://mirrordirector.raspbian.org jessie/contrib Translation-en
Ign http://mirrordirector.raspbian.org jessie/main Translation-en_GB
Ign http://mirrordirector.raspbian.org jessie/main Translation-en
Ign http://mirrordirector.raspbian.org jessie/non-free Translation-en_GB
Ign http://mirrordirector.raspbian.org jessie/non-free Translation-en
Fetched 9,670 kB in 2min 25s (66.3 kB/s)
W: Failed to fetch http://apt.osmc.tv/dists/jessie/main/binary-armhf/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.
Going to http://apt.osmc.tv/ yields the following:
Not Found
The requested URL /osmc/apt/ was not found on this server.
Apache/2.4.10 (Debian) Server at edge.samnazarko.co.uk Port 8080
Looks like the repos have gone off-line for some reason,
Looks fine here.
root@osmc:/home/osmc# apt-get update
Ign http://ftp.uk.debian.org jessie InRelease
Get:1 http://security.debian.org jessie/updates InRelease [63.1 kB]
Get:2 http://ftp.uk.debian.org jessie-updates InRelease [145 kB]
Get:3 http://apt.osmc.tv jessie InRelease [4669 B]
Hit http://ftp.uk.debian.org jessie Release.gpg
Get:4 http://apt.osmc.tv jessie-devel InRelease [4699 B]
Hit http://ftp.uk.debian.org jessie Release
Get:5 http://security.debian.org jessie/updates/main arm64 Packages [387 kB]
Get:6 http://apt.osmc.tv jessie/main arm64 Packages/DiffIndex [2023 B]
Get:7 http://apt.osmc.tv jessie/main armhf Packages/DiffIndex [2023 B]
Get:8 http://ftp.uk.debian.org jessie-updates/main arm64 Packages/DiffIndex [7900 B]
Get:9 http://apt.osmc.tv jessie/main arm64 2017-04-21-1209.02.pdiff [240 B]
Get:10 http://security.debian.org jessie/updates/contrib arm64 Packages [1138 B]
Get:11 http://ftp.uk.debian.org jessie-updates/contrib arm64 Packages [32 B]
Get:12 http://apt.osmc.tv jessie/main arm64 2017-04-21-1330.55.pdiff [267 B]
Get:13 http://ftp.uk.debian.org jessie-updates/non-free arm64 Packages/DiffIndex [736 B]
Get:14 http://ftp.uk.debian.org jessie-updates/main armhf Packages/DiffIndex [7900 B]
Get:15 http://ftp.uk.debian.org jessie-updates/contrib armhf Packages [32 B]
Get:16 http://security.debian.org jessie/updates/non-free arm64 Packages [14 B]
Get:17 http://security.debian.org jessie/updates/main armhf Packages [391 kB]
Get:18 http://ftp.uk.debian.org jessie-updates/non-free armhf Packages/DiffIndex [736 B]
Get:19 http://ftp.uk.debian.org jessie-updates/contrib Translation-en [14 B]
Get:20 http://apt.osmc.tv jessie/main arm64 2017-04-21-1331.04.pdiff [266 B]
Get:21 http://apt.osmc.tv jessie/main armhf 2017-04-21-1209.02.pdiff [243 B]
Get:22 http://ftp.uk.debian.org jessie-updates/main Translation-en/DiffIndex [2704 B]
Get:23 http://ftp.uk.debian.org jessie-updates/non-free Translation-en/DiffIndex [736 B]
Get:24 http://security.debian.org jessie/updates/contrib armhf Packages [1138 B]
Get:25 http://apt.osmc.tv jessie/main arm64 2017-04-21-1331.04.pdiff [266 B]
Hit http://ftp.uk.debian.org jessie/main Sources
Hit http://ftp.uk.debian.org jessie/contrib Sources
Get:26 http://apt.osmc.tv jessie/main armhf 2017-04-21-1330.55.pdiff [270 B]
Get:27 http://apt.osmc.tv jessie/main armhf 2017-04-21-1331.04.pdiff [265 B]
Hit http://ftp.uk.debian.org jessie/non-free Sources
Hit http://ftp.uk.debian.org jessie/main arm64 Packages
Get:28 http://security.debian.org jessie/updates/non-free armhf Packages [14 B]
Get:29 http://security.debian.org jessie/updates/contrib Translation-en [1211 B]
Get:30 http://apt.osmc.tv jessie/main armhf 2017-04-21-1331.04.pdiff [265 B]
Get:31 http://security.debian.org jessie/updates/main Translation-en [211 kB]
Hit http://ftp.uk.debian.org jessie/contrib arm64 Packages
Hit http://ftp.uk.debian.org jessie/non-free arm64 Packages
Get:32 http://apt.osmc.tv jessie-devel/main arm64 Packages/DiffIndex [2023 B]
Hit http://ftp.uk.debian.org jessie/main armhf Packages
Hit http://ftp.uk.debian.org jessie/contrib armhf Packages
Hit http://ftp.uk.debian.org jessie/non-free armhf Packages
Get:33 http://security.debian.org jessie/updates/non-free Translation-en [14 B]
Hit http://ftp.uk.debian.org jessie/contrib Translation-en
Get:34 http://apt.osmc.tv jessie-devel/main armhf Packages/DiffIndex [2023 B]
Hit http://ftp.uk.debian.org jessie/main Translation-en
Hit http://ftp.uk.debian.org jessie/non-free Translation-en
Get:35 http://apt.osmc.tv jessie-devel/main arm64 2017-04-21-1207.53.pdiff [266 B]
Get:36 http://apt.osmc.tv jessie-devel/main arm64 2017-04-21-1213.18.pdiff [266 B]
Get:37 http://apt.osmc.tv jessie-devel/main arm64 2017-04-25-1309.26.pdiff [242 B]
Get:38 http://apt.osmc.tv jessie-devel/main arm64 2017-04-26-0708.52.pdiff [268 B]
Get:39 http://apt.osmc.tv jessie-devel/main arm64 2017-04-26-1537.52.pdiff [256 B]
Get:40 http://apt.osmc.tv jessie-devel/main arm64 2017-04-26-2340.34.pdiff [251 B]
Get:41 http://apt.osmc.tv jessie-devel/main arm64 2017-04-26-2344.59.pdiff [250 B]
Get:42 http://apt.osmc.tv jessie-devel/main arm64 2017-04-26-2344.59.pdiff [250 B]
Get:43 http://apt.osmc.tv jessie-devel/main armhf 2017-04-21-1207.53.pdiff [266 B]
Get:44 http://apt.osmc.tv jessie-devel/main armhf 2017-04-21-1213.18.pdiff [270 B]
Get:45 http://apt.osmc.tv jessie-devel/main armhf 2017-04-25-1309.26.pdiff [244 B]
Get:46 http://apt.osmc.tv jessie-devel/main armhf 2017-04-26-0708.52.pdiff [271 B]
Get:47 http://apt.osmc.tv jessie-devel/main armhf 2017-04-26-1537.52.pdiff [258 B]
Get:48 http://apt.osmc.tv jessie-devel/main armhf 2017-04-26-2340.34.pdiff [252 B]
Get:49 http://apt.osmc.tv jessie-devel/main armhf 2017-04-26-2344.59.pdiff [252 B]
Get:50 http://apt.osmc.tv jessie-devel/main armhf 2017-04-26-2344.59.pdiff [252 B]
Can you try access APT from your mobile phone?
My guess is it’s a caching / CDN issue. Going to apt.osmc.tv actually took me to Index of /osmc/osmc/apt 
Seems to be OK now. The outage was happening in different regions. Strange.
