Joined up government

From the BBC:

Five offshore PFI companies paid little or no corporation tax during a five-year period despite making profits of nearly £2bn, the BBC has learned.

The five companies specialised in lending money through Private Finance Initiatives (PFI). They own hundreds of public assets including schools, hospitals and even police stations. The BBC has also learned that a small number of big offshore companies are currently on a buying spree.

I know that the whole PFI thing was a corrupt taxpayer funded giveaway and deliberately intended to make massive losses for the government/taxpayer in exchange for political party donations and cosy jobs post politics, but on a practical level, how can they get away with it?

Their 'tenant' is the government! Can't the government qua tenant just withhold 20% tax at source (like any other tenant of a non-UK registered landlord is supposed to do)? If the poor PFI companies think they've been overcharged, well it's up to them to submit proper accounts and tax returns.