The Claude Nicol Centre provides advice and treatment for people who either have symptoms of, or believe they have been in contact with sexually transmitted infections... they also provide advice for anyone with concerns about any aspect of their sexual health.
The service is free and confidential - you can refer yourself to the clinic without going through your GP.
You need to book in advance (there is no longer a walk-in clinic), by calling 01273 664721 between 9am and 11am Monday to Friday. Appointments are within three weeks for non-urgent problems and check-ups, or on the same day for urgent problems if they've got room for you.
Be prepared to take a half day off work... the check-up process is thorough and can take a few hours. Take a good book! Although this is very much a gay-friendly clinic, there's also an exclusively gay-men's clinic on a Wednesday nights (see Wilde Clinic below).