I was involved in some PFI negotiations, though not hospitals.
Back when they first came on the scene, economics meant they weren't such a bad idea - authorities didn't have up-front money to build vital buildings, but could predict roughly their income for the next 30 years, so negotiating a PFI deal like you would a mortgage was the only sensible option - the government would provide some funding if you took the PFI option for the rest.
Then certain costs went up and private funders simply didn't bid for the work (like developers deciding to stop building flats). Or there was only one bidder who would impose stringent conditions on what they were prepared to build and the amount they would get back over the 30-year maintenance contract for what services. I've heard in some cases the authority has had to start paying the contractor on a set date even when the facility isn't finished, which got clamped down on sharpish, but without a guarantee of a minimum level of payment, no private contractor would ever have got involved at all!
Thing is, however much recent governments have raved about 'competition', it's not there and it's impossible to create legal documents specific enough to cover every single aspect of what say a hospital does. There's a few new PFI hospitals where the delivery bay is too small for the standard NHS supply and catering trucks to get in, so the hospital have to pay more for everything to be delivered - but the size of doors was a level of detail not on the contract.
The 10-20% of turnover being to PFI providers sounds scary, but if a whole hospital is provided on a PFI basis, including all catering, cleaning, and maintencance staff including maintenance of all equipment, that could well represent 10-20% of the trust's budget. Hard to say if that's reasonable without more data.