A doctor who took on a gruelling charity cycle in memory of a tragic Hampstead schoolboy has raised £20,000 – despite being forced to abandon the ride.

Dr John Brook, 68, entered the 100-mile London Bikeathon on Sunday to raise money in memory of University College School pupil Joe Benett, who died in September 2012.

The 17-year-old, of West Heath Drive, Golders Green, had inhaled a cocktail of toxic gases he mistakenly believed to be the popular party drug nitrous oxide, commonly known as “laughing gas”.

He was a patient of Dr Brook’s at his private GP surgery in Oakhill Avenue, Hampstead, for most of his life.

On the day, Dr Brook had to pull out with a knee injury – after cycling the already-impressive distance of 52 miles and conquering most of the route’s major climbs, including Surrey’s iconic Box Hill.

His failure to complete the challenge did not stop him raising £20,000 for the charities Leukemia & Lymphoma Research and St John’s Hospice in St John’s Wood.

He said: “I got to halfway, having done all the nasty climbs, and my knee started hurting like hell.

“It’s a great shame, but I’m very glad to be able to raise funds like that and it’s very gratifying that I could do it in Joe’s name, that was special to me.”

The ride on Sunday was two years to the day after the teenager, who lived in West Heath Drive, Golders Green, inhaled the gases before going into a coma and dying at the Hospital of St John and St Elizabeth in Grove End Road, St John’s Wood, which is connected to St John’s Hospice.