Please forgive what may be a silly question...
If a person takes the exact same data & inputs it into the HMRC site to determine if they are (not) required to pay that tax, and they do the data entry a dozen times, can they record the answers they get & present the sites own inability to give the same answer to the same data as evidence the site is unfit for purpose?
If the site gives you anything other than the exact same answer to the exact same data every single time, then the site can't be relied upon to determine if a person has to pay or not.
Could you then present said evidence to the HMRC tax goons & say "Until you can figure out your own rules, you can stick your head in a pig if you think I'll pay this shite."?
Or is that just wishful thinking on my part? If so then I wholeheartedly humbly & sincerely apologize for my Reality Cheque having bounced. =-J