Webb's former colleague Robert Speller, who later became head of radiation physics at University College London, had moved into the field of medical physics. This encouraged Webb to consider a career in the same field and, after consultations with his friend, he applied for a job at the Royal Marsden Hospital. Early on he worked in the field of CT. Webb and his colleagues built a CT scanner by cannibalizing a radioisotope scanner. He then moved on to research in nuclear medicine, with one of the hospital's first PET scanners (named MUPPET) housed in a freight container on a lorry in the car park.