Could funding deal hinge on Barnett formula twist?

The late Lord Barnett - the inspiration behind the "Barnett floor"

It sounds like an exercise my physio might prescribe for my long-term ergonomic plan but the "Barnett floor" could be the phrase on the lips of Welsh political anoraks this Friday.

It appears to be the basis of the funding offer made by the UK government to Welsh ministers for the St David's Day agreement expected on Friday.

There's a handy guide from 2011 here. The "Barnett floor" amounts to a guarantee that funding won't fall below a certain level. It's designed to tackle the problem of convergence between public spending per head in Wales and in England.

If it sounds familiar, that may be because almost six years ago, then Welsh Secretary Peter Hain announced under the headline "fair funding for Wales" that he had "secured for the first time commitment from Chancellor Alastair Darling that the UK government will take action if Wales becomes disproportionately disadvantaged by the Barnett formula."

It even appeared in the 2010 Labour manifesto which reflected on "an historic reform which will mean that each year an assessment of the out turn under the Barnett formula will ensure that Wales is not disproportionately disadvantaged."

So - for the Welsh government - what's not to like about the 2015 version of what was Labour policy?

The biggest drawback for Cardiff Bay appears to be that the offer has strings attached. The UK government wants Welsh ministers to agree to a referendum on income tax in return.

The Conservatives see this as vital in making the Welsh government more accountable to voters but Labour have been cool on the idea without "fair funding". Would the "floor" be "fair" enough to make a referendum deal irresistible to Welsh Labour?

My understanding is that the referendum link is a Conservative initiative and there's unhappiness over it within the Welsh Liberal Democrats.

The commitment to the "Barnett floor" would also make little difference to the actual money the Welsh government gets when overall public spending is falling.

Welsh Secretary Stephen Crabb, who has cancelled a number of meetings this week, is expected to update Conservative MPs on progress this afternoon.

With barely 48 hours to go before the St David's Day agreement is due to be unveiled, we are entering what Sir Alex Ferguson described as "squeaky-bum time".

Amid concern from other parties, it is now more likely to be presented as a cross-party event, involving a possible joint appearance for David Cameron and Nick Clegg, rather than a Tory conference announcement but this - along with so much else in this story - is far from certain.