I can only speak about this particular PFI from having been in MB at the time. Clever people (like all of us here) in Town and DPA (as was) understood how ruinously expensive PFIs would be, and in particular, FSTA. But it was also clear that the Treasury would not provide up-front funding to finance a new fleet. With TB supported by GB, then GB at the helm, funding would always be an issue, and the clever accounting of PFI takes it 'off balance sheet', a bit like ignoring the APR on your maxed-out credit card. I suspect that if the plethora of PFIs are included,the MOD unfunded liability is probably far in excess of ₤37 Bn as stated.