Cache and ad blocker were excellent things to check. Here are two more to consider:

(1) Cookies previously set by sites in regular windows are not available in private windows.

You can try clearing the site's cookies in the regular window this way:

While viewing any page on the site that you can load, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that. Firefox will ask you to confirm; go ahead and confirm.

If you can't load any pages on the site, you can use the "Manage Data" button on the Settings page. See the "Clear cookies for any website" section of: Clear cookies and site data in Firefox.

(2) Other Add-ons might not be running in private windows.

You can tell by reviewing the Add-ons page, Extensions list, by checking which ones have the purple mask icon. Possibly one of the ones that is NOT allowed in private windows in causing the problem in regular windows. More info: Extensions in Private Browsing.

Along those lines, you usually can manage add-ons that are blocking content in the current page using a toolbar button. Whether that is Adblock Plus, Blur, Disconnect, DuckDuckGo Privacy Essentials, Ghostery, NoScript, Privacy Badger, uBlock Origin or uMatrix, look for its toolbar button and there may or may not be a number on the icon indicating the number of blocked items. Click the button to see what options you have to prevent content blocking on the current page.

Hopefully one of those will help.

Thanks jscher2000, It was Generat8 (add blocker) which was switched off for that site but must have been still live. All sorted now :) Bob