"Website is under maintenance" message on mobile data but not WiFi.

[Wing of a Fly]

I am using an HTC One M8 with Android 6 on EE. For work, I use this website to locate properties within a postcode: https://www1.getmapping.com/WebShop/Web/CommonPages/Main/Start.aspx

It works fine on WiFi, but when using mobile data I get the message, "Sorry, this website is under maintenance. Please check back after some time." This happens on Chrome and other browsers.

The site works fine on other people's phones, and other sites work fine on my phone. Could there be some setting or conflict causing this issue and if so, what?

 

[Wing of a Fly]

Anyone?

 

[Wing of a Fly]

If I go to Getmapping | UK aerial photography it loads, but the above link is the one which is actually useful, and bizarrely give me that message.