From January 2017 Chrome has been configured to look for user input fields (such as those you might use for entering credit card information but also things like the boxes on AB where you type your questions and answers) on any website using the non-secure 'http' protocol, and to mark it as 'Not secure'. From July of 2018 Google's policy has then been for Chrome to flag up a 'Not Secure' message on all websites that use 'http', rather than the encrypted 'https' method of transferring data, irrespective of whether there are any user input fields present.
So, in order to avoid Chrome users seeing that message, all website owners have been required to ensure that they only use 'https'. As Tuvok says, some of the pages used by Fosse Data (including their home page) are still using the older, and less secure, 'http' protocol.
Most other browsers (including Slimjet, which I'm using now) do actually mark such sites as 'Not secure' but often only by placing a label next to the URL of the site in the browser's address bar, rather than by putting an alert box on top of the relevant page.
There's nothing you can do yourself to fix the problem, other than hassling Fosse Data to get their website in order.