Media playback is unsupported on your device Media caption Union boss in "urgent need of a heart transplant"

"You hope there are many tomorrows. But you also hope you can do everything you want to do in whatever time you have."

Mark Serwotka looks well, but is desperately ill.

"Today is day 73," he says,

"But I'm not counting," he adds, with a knowing smile.

The general secretary of the Public and Commercial Services Union is in urgent need of a heart transplant.

Mr Serwotka, 53, has invited me to see him at Papworth Hospital.

The sprawling site, on the plains of rural Cambridgeshire, began life a century ago as a convalescence and treatment centre for people with tuberculosis.

But for the past 50 years it has been a global pioneer in the treatment of heart and lung disease.

The very operation Mark Serwotka is waiting for was carried out successfully for the first time in the UK here in 1979.

"Since I have been here, I have had two very close false alarms, where I have been prepped for surgery," he says.

"In the hospital gown, shaved and literally ready to be taken down for a heart transplant, when problems were found with the donor organ, under surgical inspection, that meant it had to be cancelled at the last minute, and in one case that was within 20 minutes of me going to surgery."

Union firebrand

Image copyright Roger Donovan Image caption Mr Serwotka had been fit and healthy before contracting a virus that damaged his heart

Mr Serwotka is a veteran of the trade union movement, a fiery left-wing warrior so often associated with tub-thumping denunciations of what he sees as the injustices of cuts in government spending.

But, as we stand together, just outside the hospital's cafe, in the failing light of a cold November afternoon, he is reflective and emollient, as he recalls events six years ago.

"I lived a very fit and active life. And I contracted a virus. We think it was contracted whilst taking my dog for a walk in the woods. He came back smelling of something foul and so I washed him down.

"The next day my face and my legs had swollen grotesquely. It looked like an extreme allergic reaction.

"A week later I got rushed to hospital with a heart rate of 220 beats per minute.

"And after six weeks of investigation it was established I had contracted a virus that had caused my heart to massively inflate and swell, and when the swelling went down the heart had been irreparably damaged."

Image copyright Andrew Aitchison Image caption Mr Serwotka is a leading figure on the left

Mr Serwotka was diagnosed with heart failure and within two years, he couldn't walk even 50 yards.

He was fitted with what is known as a left ventricular assist device. It is a heart pump, complete with battery pack and plug socket.

"I have an electric cable that comes from my heart, out through my abdomen, into a power pack, which is powered by batteries, but if I am in bed, I plug it into the mains. If I am driving the car, I plug it into the cigarette lighter."

Then in August, Mr Serwotka developed blood clots.

He was admitted to Papworth Hospital and has never left.

As we chat, the wind occasionally catches what he calls his "lucky windmill."

Constant medication

It was handed to him by a member of staff when he was admitted, and is attached to an intravenous drip, on wheels, that he pushes around everywhere he goes.

"It's delivering a blood thinner. It is on 24 hours a day. And it is thinning my blood to reduce the probability of a clot returning.

"If the blood thinner stopped, obviously the danger of the clot reforming in the pump, which can be fatal, can be a very serious concern, and it is because I am on this that I can't leave the hospital.

Image copyright PA Image caption Papworth has a long history of excellence in heart care

"On a day to day basis, I exercise, I am working, and I live quite an active life in the hospital, but I have to remain until such a time as a suitable heart donor is found.

"The clot means it is a life-threatening situation that only a heart transplant can resolve."

Work, he says, can never take his mind off the life-defining situation he finds himself in, but it has stopped him going "stir crazy".

The Shadow Chancellor John McDonnell had visited the day before I did; his team visit weekly so he can carry on with his work.

"The reality of my situation now is without a heart transplant, I won't be leaving the hospital," he says.

"I have seen five fellow patients have transplants while I have been here, and leave in an extraordinarily fit condition, so you can inspired by the fact that you see how it works."

Mr Serwotka, for all the bombast of a trade union heavy hitter, has clearly done a lot of quiet thinking.

Image copyright PA Image caption Union leader Bob Crow died of a heart attack at 52

"When Bob Crow died, I remember very vividly, one of the RMT banners in his name was about living for today. You shouldn't regret having not done things that you want."

Mr Crow was the leader of the RMT. He died in 2014 of a heart attack, aged 52.

A few yards away, Mr Serwotka's wife and son wait patiently at a nearby table, generous in allowing me time with him during their visit.

"Whilst I hope to have a long life with my family and make a big commitment to the British labour movement, the reality is you never know what is around the corner and therefore you shouldn't have any regrets," he says.

"I have been been in a very serious life-threatening condition about three times in all this.

"So you clearly confront your own mortality, and I have always believed it is harder for your family. They are the ones who have to deal with that and the aftermath.

"All of the statistics say that the average life expectancy from a successful heart transplant is 13 years. That means the statistics tell me that my life expectancy will be considerably less than many.

"But there are people who have lived 30 years. I am determined not to live every day worrying about what tomorrow brings, but to make the most of today, and you hope there are many tomorrows."