The shadow chancellor John McDonnell has pledged to bring at least some private finance initiative (PFI) contracts “back in-house”. How big a commitment is this? The PFI is a fertile source of Very Big Numbers – McDonnell himself refers to a figure of £200bn for payments “over the next few decades”, while John Appleby of the Nuffield Trust is quoted by the BBC giving a figure of £56bn for NHS projects alone by 2048. That may suggest Labour is promising transformative changes to public spending and public services, and will doubtless feed into some highly charged headlines tomorrow. But the expenditure and policy implications may well be of more modest dimensions.

For a start, as Labour’s press release makes clear, the pledge falls far short of taking all or even any PFI contracts back: “Labour will review all PFI contracts and, if necessary, take over outstanding contracts and bring them back in-house, while ensuring NHS trusts, local councils and others do not lose out, and there is no detriment to services or staff.” No less importantly, McDonnell’s choice of words – “We’ll bring existing PFI contracts back in-house” rather than “taking PFI schools and hospitals into public ownership” – may suggest that it is the services provided under PFI contracts rather than the physical assets that he is talking about taking back under public control.

Some context is needed here. PFI contracts typically involve both the construction or refurbishment of a physical asset such as a hospital, and the provision of maintenance (and sometimes other) services over an extended period – typically 30 to 40 years in the NHS. Including services in the contract was primarily a way of keeping the borrowing for construction costs off the government’s balance sheet. This famously allowed New Labour to invest heavily in infrastructure without affecting public borrowing figures, at the cost of higher financing costs over the long term and of locking providers into very long-term relationships with contractors for the provision of services. PFI service provision has been a major source of discontent in the health service (as in the stories of NHS trusts being forced to pay extortionate charges for the changing of a lightbulb).

Taking some of these maintenance services back in-house would be a different matter to taking over the ownership of the physical assets. The capital value of existing and planned PFIs is some £59.4bn (although some of this has already been paid off by the public sector) and the contracts are structured to protect investors. It is unlikely that government would save any money by taking them into public ownership and it might well wind up out of pocket due to penalty clauses. Taking back services that have to be paid for anyway might (I stress might) actually save money. And of course it would be popular with public-sector workers, a key part of Labour’s constituency.

So is McDonnell’s pledge a major step on the journey to socialism or a modest reform to a deeply unpopular programme? We’ll need to see more detail about what’s being proposed. Whatever the plan, the devil is likely to be in the detail rather than in the Very Big Numbers. PFI was introduced as a complex new form of contracting designed to get around public accounting rules. Extracting the public sector from the fiendish complexities of PFI contracts, even just to take services back in house, is likely to be a difficult and costly task. Setting up a review doesn’t sound very exciting but it’s probably a sensible way to proceed.

• Declan Gaffney is a freelance policy consultant