First Aid, will a police caution for assault 5 years ago prevent me from joining St John ambulance?

No, not necessarily. Obviously you'll need a CRB check to join the ambulance service so it will show up, but you might be OK if you can convince them that you are sorry, it is a one-off (if it is your only brush with the law), and also you can stress that it is just a caution so it was considered not serious enough to be a conviction.

You will have some explaining to do but it does not automatically stop you from applying or joining.

Do you mean about the CRB check?

I was a St John's Ambulance volunteer in my workplace years ago when my company sent me on a course with them and when it was easy to join and never had CRB check and things. But now, I think it all depends on the 'nature' of the assault possibly. St John's Ambulance are always in desperate need of volunteers and I think it's great that you want to join it, but if the assault charges were say a fight or a scuffle in the street when you were young, they might just disregard it. 'Assault' covers a huge area in law, and there are degrees of assault minor to major. If your charge was of a lesser nature, then perhaps you will be allowed to join. I don't know for sure though.