There was a mixture of joy and trepidation for Watton parents Callie and Simon Blackwell as they were told they could take their 13-year-old son who is battling a rare combination of cancers home to Norfolk for the bank holiday weekend.

After spending almost a year in hospital, Deryn Blackwell's family said the relief they felt that they could travel back to the county for three precious nights together was 'overwhelming'.

It has been more than six months – almost 200 days – since Wayland Academy student left Norfolk on Valentine's Day to travel to Bristol for life-changing treatment at the Bristol Royal Hospital for Children.

Today, the teen penned a moving letter to the EDP about his experiences, in which he talks of how much he misses his family, friends and BMX. After being diagnosed with leukaemia aged 10, Deryn developed Langerhans Cell Sarcoma as well – one of only six cases in the world.

Thought to be the only person to have both conditions at the same time, Deryn had been treated in Addenbrooke's Hospital in Cambridge since September 2012 and only after a few weeks back a home was taken to Bristol for specialist care. Doctors there had hoped his treatment, which included total body radiotherapy and targeted radiotherapy to his throat before the operation, a long period of isolation after it, and a dose of 21 drugs a day would tackle the cancer.

Since his transplant in March, Deryn's mum, Callie, says the family have been on a 'roller coaster-ride' of emotions.

Doctors had said it looked as though Deryn was virus free and his bone marrow had grafted weeks after the procedure and family and friends had hoped Deryn would return to Norfolk this summer after making a good recovery. But last week doctors delivered news that a virus had returned and that it was difficult for Deryn's body to fight it as treatment to kill the virus had started to kill off the good work the bone marrow has done.

Deryn has lost more than 22lbs in the six months he has been in Bristol and his mum says she is trying every recipe she can to keep him well.

But this weekend good news arrived yet again that the family, dad Simon, younger brother Dylan, and mum Callie, could together travel home for the bank holiday weekend.

'We try not to get over excited about anything now', his mum, who no longer works so she can dedicate her time to Deryn, said.

'It is so lovely to all be at home, but I do worry Deryn will want to come back all the time when the doctors might not allow it. We have become accustomed to this change now – it's our normality. We just have to keep looking forward and don't look back.'

Since the start of Deryn's treatment in Bristol the family have split their time between the south west and Norfolk, with one parent staying in a house donated to the family for their time there by a local charity, and the other in Watton with Dylan, while he is at school. Despite the house being provided for free by a local charity, the family estimate they still spend an extra £500 a month on supplies, including food, internet and petrol.

The family will travel back to Bristol today and Deryn will continue his treatment as an out-patient in Bristol.

Tweet Deryn at @DoEveRYthiNg and follow his mum's blog at www.doeverything.org.uk.