Every autumn the health service waits for a Budget announcement as though it were the only mechanism that decides how much money reaches wards, ambulance trusts and capital projects. This year the more consequential decision may already have happened by the time John Healey stands up on 28 October, or is about to happen a fortnight before it, inside a Bank of England meeting room that has nothing to do with health policy at all.

The Monetary Policy Committee will soon set its pace for gilt sales for the year ahead, part of the unwinding of the more than £800bn in government bonds the Bank bought during successive rounds of quantitative easing. Thirty-year gilt yields touched their highest level since 1998 last month. Ten-year yields remain close to the post-financial-crisis peaks reached in April. Economists and bond investors are now urging the Bank to stop actively selling gilts into this market altogether, arguing that the sales themselves are adding to the very borrowing costs the government is straining under.

The mechanics matter because of who ultimately pays. Under the arrangement struck between Mervyn King and George Osborne, the Treasury absorbs the losses from this unwinding immediately rather than spreading them over time, as most other central banks choose to do. Official estimates now put the eventual bill as high as £125bn, a sum roughly equivalent to two years of the Department for Education's entire budget. Each gilt sold at today's depressed prices crystallises part of that loss there and then, competing for space in the same fiscal envelope from which departmental settlements, including the Department of Health and Social Care's, are drawn.

This is not an abstract concern for a health service still absorbing the aftershocks of last month's discharge figures, where medically fit patients stuck in hospital beds hit a new record, or still working through the funding implications of guaranteed-hours contracts under the Employment Rights Act. Every one of those pressures eventually resolves into a question about how much headroom the Chancellor has to spend, and that headroom is shaped as much by debt-servicing costs as by growth forecasts or tax decisions. A Bank of England that keeps adding supply to an already strained long-dated gilt market pushes up the yield the government pays on its own borrowing, which narrows the margin available for everything from NHS capital schemes to public sector pay settlements agreed under this year's less generous fiscal rules.

There is a reasonable case that the Bank's independence from fiscal considerations is precisely the point, and that Governor Andrew Bailey is right to want the balance sheet capacity to respond if another crisis demands fresh stimulus. However, even the Bank's own research, which was released in the Monetary Policy Report last month, now acknowledges that active sales have increased long-dated rates by up to 30 basis points. A year ago, this acknowledgement would have been unimaginable. When a technical mechanism designed to sit quietly in the background of monetary policy starts visibly shaping the cost of government debt, the separation between monetary and fiscal policy becomes harder to defend on principle alone.

For NHS leaders and policymakers, the practical takeaway is less about the economics of quantitative tightening than about where genuine fiscal risk now sits. A slower pace of gilt sales, which several analysts expect the MPC to vote for in September, would ease some of that pressure before Healey's Budget arrives. A decision to press on regardless would tighten it further. Either way, the room Cooper has to fund the community mental health expansion already announced, to address discharge delays, or to fund whatever emerges from the ongoing NHS Oversight Framework review, will owe more to a vote at Threadneedle Street than to anything argued in Parliament this autumn. Health policy has rarely been quite so exposed to a decision so far outside its own building.