I did not realise how many of these trials are on offer. Here is a typical FAQ which suggests to me the monies received are not income, will not be taxed and should be treated as capital.
“Do volunteers get paid?
Most CRO’s provide incentives to healthy volunteers to compensate you for the amount of time you spend participating in a trial, and reimburse you for any inconvenience and travel expenses incurred along the way. Generally speaking, Phase I clinical trials will pay you for participating, while others will not. In some trial studies, you may be reimbursed for expenses associated with participating in the research such as travel costs, child care, meals, and accommodations.
As a healthy volunteer, you may be compensated £80-150 per day for your inconvenience, depending on the trial and unit. Study-related medical care and study medication(s) or treatment(s) are generally provided free of charge.
The trial sponsor and investigator jointly make a decision about how much volunteers are paid. An ethics committee then reviews the level of payment to make sure it is appropriate. The amount you could receive for participation is largely in proportion to the amount of time required of you to be in the trial. Some trials may require you to visit a site for just two hours, stay overnight or live in for 2 weeks. Hence the total amount you can receive as a healthy volunteer could be as little as £20 or well over £2,000. For patient volunteers one of the possible benefits is gaining access to promising new treatments before they are widely available”.