There is a small but steady flow of petty cash through most practices. Control of this is very important. Most doctors will know of the type of work which earns a few pounds at a time, and often as cash. This includes private sick notes, cremation fees, some medicals, the occasional immunization certificate and so on. It is very easy to lose track of this income, but the Inland Revenue will be aware of it. Small indiscretions they will overlook, but they will maintain full records. If however you get into a major investigation, they can go back 7 years to find if you have erred from the straight and narrow. If they find that you have not been 100% accurate, they can go back another 7 years. They can do it over and over again. It is therefore foolish not to do the job accurately and with care. You have enough burdens without having the Inland Revenue investigating you.