I am a 23-year-old PGCE student about to start job hunting in London. I have a police caution from three years ago for cannabis possession; it appears on my enhanced disclosure and will be there for another two years.

When should I disclose this: before interview, or see if they offer me the job and then tell them?

Disclose it on your application form. If you have worked in schools through your training as a PGCE, get those schools to endorse your abilities as a teacher, since what you want is for a future employer to weigh the record against your more recent performance. You should also show that you are able to communicate with young people who have experimented, or are considering experimenting, with illegal substances. If you fail to disclose any relevant fact, you may be committing a further and more serious offence.