“It was really hard, there were tears, there were points when we were like ‘oh, God’, but we finished it and it feels amazing to have done it,” Miss Nichols, a 21-year-old student doctor, told the PA news agency shortly after finishing.
Since July 23, Miss Nichols and Mr Bates have completed an array of events including skateboarding, climbing, boxing, weightlifting, shooting, show jumping and rhythmic gymnastics.
Rather surprisingly, though, Mr Bates cited trampolining as his toughest event due to a problem he had with his back.
“I never want to see a trampoline again as long as I live,” he said. “Give me two marathons but no more trampolining.”
The couple held hands and were cheered by onlookers as they finished the very last leg of their journey on Sunday afternoon, minutes after the closing ceremony in Tokyo.
Mr Bates said it was “just super special” to complete the event in Weymouth, where his brother had lived.
“It just felt like coming home and the reception at the end was just astonishing, all of our family there cheering us home,” he told PA.
The pair have also received the backing of several real Olympians, past and present, during recent weeks – including silver medal-winning athlete Keely Hodgkinson and gold-winning kayaker Liam Heath.
It was not a smooth ride through the events, however, with Mr Bates thrown from his horse during cross country and Miss Nichols having to be rescued while windsurfing due to a phobia of fish.
The couple are raising money for the Motor Neurone Disease Association, which helped Spencer during the final years of his life.
Mr Bates, 51, a window cleaner, said: “It is so hard to watch someone you love living with motor neurone disease. It’s 10 years since Spencer passed but I still think about him every single day.”
The couple’s efforts have drawn donations from thousands of people across the world.
“It gives us an awful lot of pride but also so much motivation,” Mr Bates said.
“We woke up yesterday and our bodies were in pieces, but we read a few messages from these people and we got up, dusted ourselves off and got out.”
As well as financial support, dozens of sports clubs offered training and equipment to help them complete their challenge – while the University of Bristol provided free access to its facilities and assistance from performance coaches.
Mr Bates and Miss Nichols said they would have a “few drinks” with friends and family to celebrate before taking a bit of time to recover.
However, their fundraising efforts are far from over.
“Watch this space, it’s not the end of Spennylympics,” said Mr Bates.
The couple’s fundraising page for the Motor Neurone Disease Association can be found at https://www.justgiving.com/fundraising/spennylympics