By Chris Hogg BBC Health Correspondent

What chance is there that the new NHS pay deal will fall at the first hurdle?

Agenda for Change covers more than a million health service workers.

It took four years of negotiations between Department of Health officials and representatives from 17 staff side organisations to hammer out a deal.

Agenda for Change covers more than a million health service workers

Both the government and the unions are keen to get across to the staff involved that it is not just a new pay deal but a completely different pay system.

Staff will be matched to a new job description and placed on a new pay scale.

There will be more opportunity for training. Employers will be able to pay more to attract staff in short supply.

Pay rises

Basic pay will rise by around 3.2% in each of the next three years.

Most staff will benefit from a re-grading worth an average of 5.9%.

So now the hard sell is on.

Between now and April, health service workers will be balloted on whether or not to accept the deal.

But the concern is that some might find it hard to sell it to their members.

Some of the unions involved were unhappy at the way the Department of Health handled the announcement that a deal had been reached.

They complain that ministers used it as a way of bashing the fire-fighters.

Health Secretary Alan Milburn did a round of media interviews claiming that his deal showed how the modernisation which the fire-fighters were so opposed to could benefit public sector workers.

The distinction between fire service modernisation, which their union feared meant job cuts, and NHS modernisation, which health unions understood to mean recognising and rewarding changes in the workforce which have already taken place, became blurred.

And the three-year pay deal, which was again widely publicised at a time when negotiations with the fire-fighters were faltering, was a bitter pill to swallow.

Winners and losers

Then there is the question of winners and losers - always difficult when you are creating new pay scales.

One nurse consultant I spoke to told me that being asked to vote on whether or not to accept the deal felt a bit like trying to decide whether or not to take out a bank loan without knowing the interest rate.

The flexibility of the system means that although "typical" case-studies will be circulated to try to give people an idea of how they do under the new system, in reality for many in the NHS it will be hard to work out how much they will benefit.

Those who work unsociable hours or who live in high-cost areas will benefit from several different types of bonuses - the effect of which will be different for almost every employee.

The government has made clear that those workers who do "lose out" financially when their jobs are re-designated will be compensated so that their pay packets don't shrink.

But that might not be enough to stop some elements of the workforce who might feel their allowances are being downgraded voting against the deal.

So for the union negotiators. who have spent many months working on what they think is the best possible deal they are going to get from Whitehall, it will be a question of trying to persuade their members to look at the bigger picture.

As one union representative told me night pay bonuses will be cut from 30% to 25%.

But, whereas at the moment a member of staff who swaps to a day shift to undergo some training loses that allowance, under the new system they will not.

Union support

The Royal College of Nursing's executive has backed the deal - probably not that hard a decision based on the fact that most of their members will be net-gainers.

For Unison, which represents almost half of the members being balloted, it is harder to call.

Department of Health officials will not be drawn on what might happen if one of these major unions...rejects the deal

The union represents almost half of the NHS staff covered by the deal.

Agenda for Change will benefit many of its lower paid female members as it will remove a lot of the sexist anomalies which mean they can end up being paid less than their male colleagues for doing the same jobs.

Many of their staff who work in ambulance services have also done well, but others who work as cleaners or porters on permanent night shifts may not be so happy.

Department of Health officials will not be drawn on what might happen if one of these major unions, Unison, the RCN or indeed the GMB which represents many low paid NHS staff rejects the deal.

All they will say is that the negotiators will hold a meeting once the final ballot is completed to see where they go from there.

The unions' negotiators are taking nothing for granted. As the rejection of the consultants' contract showed, staff in the NHS are not always guaranteed to follow the lead of their union leaders.

Some disgruntled staff might want to push rejection as an opportunity to give the government a bloody nose.

The problem for the unions' negotiators will be persuading them that this is the only offer on the table.