init.json endpoint causes access denied

[mlynngh]

I use this end point for my TV channel listing website in PHP:
http://epgservices.sky.com/tvlistings-proxy/TVListingsProxy/init.json

At some point between July 6th and August 12th 2020 this has changed and is causing Access Denied errors:
Access Denied
You don't have permission to access "http://epgservices.sky.com/tvlistings-proxy/TVListingsProxy/init.json" on this server.
Reference #18.64e41602.1597320682.442cd62

It seems this requires authentication but how do we do this due to the fact this API is undocumented?
I contacted the Sky Community about this but they archived my question and did not provide an answer.