The firms behind one of the first hospitals in Britain to be built and operated using private money have more than trebled their investment in six years.

A National Audit Office report into Norfolk and Norwich University Hospital NHS Trust shows that Octagon Healthcare, a private consortium, gained £115m when the PFI scheme was refinanced in 2003.

Although it shared £34m with the trust, it kept a profit of £81m. The document also suggests that the trust is paying £5m a year more than it should have done if the deal had been better negotiated in 1998.

The trust pays £37.8m a year to Octagon, which the NAO suggests should be lower. The report says that the trust changed the requirements in the closing stage of signing a deal, which meant that the consortium was able to increase its fee without competition from others.

The NAO report points out that construction costs have risen for more recent PFI deals, which may offset the return from refinancing.