URL entered results in URL contacted failing with 403 error and a change in shown URL

I am not sure where this problem lies.
I enter a URL - https://fictitious.co.uk
That should go to logon page.
Instead it results in a 403 error with the address bar now displaying a modified URL - https://fictitious.co.uk/anotherpage
I have cleared cookies and history, I have tried this with Google and Bing, and also on my tablet and PC - all do the same. I am using up to date Windows 11.

Who is at fault - me or fictitious.co.uk?

More than likely the site itself is having issues, as the redirect is to a page on the correct domain, but it could be the result of a malicious actor (attacking the site, not you).

One thing - when on mobile, were you on your personal Wi-Fi or on the mobile cellular data connection? It's a (very small) possibility that it could be your router, as well, though I'd doubt it, as it is redirecting to a page on the same domain. Still worth exploring.

I was using my WiFi, but like you I doubt it is a router issue.
The site itself is AG Intl
At present I get the impression that the St Bees Priory database, could be the biggest one they currently have in the UK, certainly biggest in Cumbria. I was involved in some of the early work transferring data from the local web site I produced to their national one. As a result they are using it as a bit of a test area. This is not the first time I have obliged by breaking something on their developing site. Posted question here to try to make sure it wasn't my fault!