I've been running my own UK Private limited company for a few years now. Because I'm cheap, I do my taxes myself. Usually from a location that would be considered glamorous by UK standards. As such calling the support line isn't really an option.
In recent years I've had a number of issues downloading the CT600 form. If you want to file independently you have to do it via this form, there is no alternative. This of course creates an issue if your nearing the deadline, and can't access the form.
If you happen to be having issues I have a single tip for you. Private browsing mode. Works for me every time.
Happy Tax filling day.