Is it just me?
I thought EVERY company put into employment contracts that all email/documents/inventions etc etc etc belonged to the company and most include that email is not for personal use. Not sure how you get from there to not being allowed to access staff emails if they were sent to a company email account since the fact that they were sent to a company email account would surely put them on the 'company ownership' side of the equation.
I know there was a European Privacy thing a while back which said that a company could not read personal emails sent to your company email account and would have thought this strengthened the aspect that the mail account belongs to the company. From the sounds of it, these emails contain information relating to the financial status of the company and so surely HMRC would insist that the company could produce them.