StudentGems ad hoc projects can be invaluable. Add it to your CV so you can demonstrate your skills and knowledge to temping agencies.
There are obvious parallels. StudentGems jobs are temporary. They are paid. They require you to have good business standards and commercial awareness. These are all the things temping agencies will be looking for.
As soon as the summer exams are over, go to a couple of agencies and see which ones you like best. Sign up early so they know they can count on you over all those long summer holidays. The great thing is you can be available when all the regular office workers are flying off for 2 weeks in the sun!
Office Angels have some guides to temping here.