The British journalist held in Russia on piracy charges following a Greenpeace demonstration against Arctic oil drilling has told his family that he is coping with his detention by exercising, writing and reading Jane Eyre.

Kieron Bryan, a 29-year-old freelance videographer who was hired by the environmental group to document last month's protest in the Pechora Sea, said he was doing his best to look after himself in "very basic conditions".

In a 20-minute phone call from the Murmansk detention centre where he and the rest of the "Arctic 30" are being held, Bryan said he was trying to remain optimistic.

"When he finally called on Saturday it was like all my Christmases had come at once," said his mother, Ann Bryan.

"Kieron sounded strong and calm, and told me he was trying to stay positive, though that was sometimes hard due to his not being able to communicate regularly with other people.

"He found the hardest things to cope with were the uncertainty of his future and occupying his time in his cell."

To keep himself busy, she said, Bryan was writing – "letters, thoughts, anything that comes into his mind" – and reading. "He's now finished the only book he has been able to get hold of so far, Jane Eyre, and said he would very much like some more."

Ann Bryan said that her son had been cheered to learn of the support his case had attracted but saddened at the thought of missing his grandmother's 90th birthday this Sunday.

"He mentioned his nan's forthcoming 90th birthday and told me to give her a big hug from him and to say sorry he couldn't be there," she said.

Although a big party had been planned, his grandmother has decided she doesn't want to celebrate without him.

Bryan also managed to phone his girlfriend, Nancy Thorburn, on the same day.

After weeks of avoiding the London underground and anywhere else that might interfere with her mobile phone reception, Thorburn said she had been delighted when the call finally came as her mother was driving along the motorway.

"As soon as I said Kieron's name she pulled over into the nearest layby so that we wouldn't lose the phone signal," she said.

"At one point, I had my back to the car and my shoulders were shaking – my mum thought I was crying, but Kieron was trying to cheer me up, and was telling me things to make me laugh."

Despite maintaining a sense of humour, Bryan admitted that life in the centre was difficult: the noise at night had led him to try to sleep in the afternoons, while the food was "not ideal".

He told his girlfriend that although he was keeping fit by doing press-ups and sit-ups he was confined to his cell for 23 hours a day and missed being outside.

"Kieron is resilient and is doing his best to look after himself in very basic conditions," said Thorburn. "He may have been trying to reassure me, but I think he was being honest."

Bryan is one of six Britons who have been in custody since the Greenpeace ship Arctic Sunrise was seized by Russian authorities on 19 September.

Last week another of the British detainees, Alexandra Harris, wrote to her family to tell them of her fears for the future.

"I have moments of feeling panicky, but then I try to tell myself that there's nothing I can do from in here and what will be, will be, so it's pointless worrying," she said. "But it's hard. Surely my future isn't rotting in prison in Murmansk?! Well, I really hope it isn't.