WP site showing '403 – Forbidden' on windows server when no 'www'

Hi

I’ve set up a website for a client on their Windows web server (which I know very little about).

The website works fine when you enter wwww.domain.co.uk and redirects from https://www.domain.co.uk to www.domain.co.uk ok, but when you enter https://domain.co.uk or domain.co.uk the site throws up a 403 – Forbidden: Access is denied error.

Does anyone know how I’d go about fixing this please?

Thanks