The BBC has been investigating overtime payments made to consultants by hospitals.
We have used a combination of Freedom of Information requests to 186 trusts and health boards across the UK - 140 provided answers to at least some of the questions - and then followed that up with requests for extra information from more than 20 of the organisations, carried out interviews with hospital managers and doctors, and cross-referenced the findings with other sources of data.
The investigation has found wide variation in approaches to overtime - and a rising cost from paying it.
How does consultant overtime work?
A full-time consultant is expected to work 10 shifts of four hours a week. The average basic salary for that is £89,000 a year in England.
Payments for overtime are negotiated locally by hospitals. Some consultants agree to do extra shifts under the terms of their contract - so-called plain time.
But others are paid at a higher rate for extra work - and it was on this that the BBC investigation focused.
Hospitals take a variety of approaches when it comes to these payments. The highest rates tend to be paid for routine work, such as knee and hip treatments.
These are paid under a system called the waiting list initiative, and commonly, hospitals pay around £600 for a four-hour shift for these. But the BBC has found evidence of payments climbing to around £1,000 for some, once supplements are added on.
Many hospitals have negotiated lower rates for more urgent work. These are commonly around the £300-a-shift mark.
How much do consultants make in overtime payments?
Our findings suggest the average payment is just over £13,300 per consultant per year for those who receive them.
The most made in overtime payments was by a consultant at Lancashire Teaching Hospitals, who received £375,000 in 2015-16. That was on top of the individual's basic salary.
The trust has refused to name the doctor or reveal what speciality he or she works in - and despite repeated attempts, has refused to divulge more information about the case, except for saying that they have worked "significantly more hours than usual at an enhanced rate" because of a shortage of consultants and high demand.
That was the highest payment by some way, although only half of trusts and health boards responded to the question about the most a consultant had made.
The next highest payment was £205,000 to a consultant employed by East Kent Hospitals.
But it does appear high payments are widespread - two out of three trusts and health boards had paid at least one consultant more than £50,000 last year, with one in four paying £100,000 or more.
Surgeons, radiologists, urologists, anaesthetists and gastroenterologists were the specialists most likely to get the high sums.
How can consultants earn so much?
While payment for premium overtime shifts is commonly around £600 for a four-hour shift, there is a wide variation in how overtime is rewarded. Even within the same hospital rates can vary enormously.
Every trust the BBC approached for detailed information on how individual consultants had earned the large sums refused on data protection grounds.
But talking to people who work in the health service we have been able to piece together some of the ways doctors can rack up six-figure salaries.
It goes without saying, they do lots of hours - some are regularly working 20 hours a week or more in overtime.
The most experienced doctors are able to command extra supplements on top of the normal high-rate overtime payment. This can bring the payments to around the £1,000-mark. Doing back-to-back four-hour shifts, it is possible in theory to make £2,000 a day.
Remember though, not all consultants work full-time, so the overtime may not be on top of a full 40-hour week.
How common are these payments?
From the information we have been provided it is hard to get a really clear idea of this.
It would appear as many as half of consultants may get higher rate payments. Some 98 trusts and health boards provided full information about numbers. This showed more than 13,000 consultants had received high-cost overtime payments - about half the workforce in these hospitals.
There could be some double-counting as some hospitals had used doctors from other hospitals to do the work, while others had offered waiting list initiative shifts to locums as they could not fill them with their own doctors.
But hospitals told the BBC the overwhelming majority were consultants employed by them themselves.
And data from other sources, including the Health and Social Care Information Centre, suggests somewhere approaching half the workforce would be a reasonable estimate.
How much do these payments cost the NHS?
A total of 114 trust and health boards answered the question about the total spent on high-cost overtime in full over the three years.
That is about six in 10 of the total in the UK so we only have a partial picture. Among those that responded, spending reached £168m in 2015-16 - a rise of a third over the past two years.
If that spending was replicated across the other hospitals the total would be close to £300m - that would be enough to pay for about 3,000 consultants or more than 12,000 nurses.
But to put that in context £6bn is spent on consultant pay in England so it looks like premium rate overtime accounts for less than 5% of spending in this area.
Why are hospitals paying them?
The hospitals to whom the BBC spoke commonly cited a growing shortage of consultants. In Scotland, the vacancy rate for consultants is between 6% and 7%.
Official figures are not kept in England, but anecdotally it seems a similar shortage is being experienced.
In some of the trusts with the highest rates of overtime payments the vacancy rate is in excess of 10%.
Demand for care is also cited as a cause. It is well documented that patient demand is rising and hospitals are under pressure to hit their waiting-time targets.
Hospitals have also pointed to the rising pressures in emergency departments. This causes routine treatments to be postponed - and that is the care that is the most expensive to arrange overtime for.
Is the government right to target the consultant contract?
Ministers in England want to change the consultant contract and remove the opt-out they have for working weekends when it comes to non-emergency care as part of their drive for more seven-day services.
Talks are under way, but it is likely to be the autumn before there is any resolution.
While the focus of seven-day care is not really on the routine elements of treatment, negotiators believe removing it will help create a fairer system, especially if they manage to get the British Medical Association to agree to a more standardised system for extra work.
That in theory would free up money, which, of course, would help with extending services.
But the BMA argues the problems with shortages would still exist - and hospitals would be faced with the choice of paying overtime or contracting work out to the private sector. They are already under pressure to reduce the use of locums so that is not really an option.