Sparring on spending: The latest round in the NHS funding row

It is unclear how much of NHS England's extra cash will go to Welsh Government coffers

It took just 84p to reignite the often unedifying spat over health funding in Wales and England.

Politicians had been talking about the biggest cash injection to the NHS since austerity began.

But at Prime Minister's Questions last week Theresa May repeated the 84p figure, first deployed by Welsh Secretary Alun Cairns two days earlier.

For every extra pound spent on the English NHS between 2011-12 and 2016-17, the Welsh Government has spent 84p.

The sums add up. Since the first full financial year after David Cameron moved into Downing Street, health spending has risen by £290 per head in England and £244 in Wales.

Theresa May has said the NHS is her government's top priority

But this overlooks the fact that England is playing catch-up.

Per capita health spending in Wales is still higher - £2,233 vs £2,169 at the last count.

What's more, who's winning the race depends on where you put the starting line.

The data come from Treasury reports (see here and here) covering five-year periods. The Welsh Government would rather concentrate on the last report, during which time spending per head grew 14% in Wales and 13% in England.

Early in the decade the Welsh Government froze some aspects of NHS funding, hoping to spare social care budgets from deeper cuts. The charts show the effect of those decisions - and how spending later picked up again.

Officials in Cardiff try to cast doubt on the UK government's analysis with a pretty broad statement that because the Treasury "often makes minor changes to methodology… it is not always strictly accurate" to compare the reports.

This all flared up again because of the £20bn a year promised to the NHS in England by 2023 by Theresa May.

Initially we were told this would increase the size of the Welsh budget by £1.2bn a year. Later the Treasury said it would be £1.37bn.

So why won't Labour do as the Conservatives want and commit to spend that money on health?

Because they're looking at the calendar as well as the spreadsheets.

We'll have to wait until the chancellor's budget in the autumn to find out how what's really happening.

Philip Hammond has confirmed taxes will rise to pay for this 70th birthday present for the NHS, but what if he also cuts funding for Whitehall departments whose functions are devolved to Wales, such as transport or local government?

That would mean what Mrs May gives Wales with one hand, her neighbour on Downing Street takes away with another.

Hence the Welsh Government can say, with some justification, that £1.37bn is a gross contribution, not net.

Finance Secretary Mark Drakeford does not know how much extra cash will come his way

Thomas Pope, of the respected Institute for Fiscal Studies, says: "What else is going to happen to the public finances to pay for this?

"It's almost certain the Welsh Government will get some more money as a result of this. How much more we will have to wait until the autumn budget to see what the chancellor does."

He describes the £1.37bn as an "upper bound" on what Wales could get, adding: "These are relevant numbers, but they can't be thought of as true numbers or anything the Welsh Government can plan on just because it will depend what happens in the budget."

Wales' Finance Secretary Mark Drakeford will publish his next budget around the same time as Mr Hammond, but he cannot yet add in the proceeds of the prime minister's largesse.

The Welsh Government says any extra funding it eventually gets will have to help pay for lifting the NHS pay cap.

In the meantime, don't expect either side to depart from their scripts.