I am from the UK & was arrested and cautioned for cannabis possession in 1996. Will this stop me getting a visa waiver for entry to the US ?

Potentially, yes.

If you were under the age of 18 when the incident happened then you can answer No to the quesiton about arrests and convictions.

This article explores the issue.

http://britishexpats.com/articles/uk-police-cautionswarnings-and-us-immigration-law/

If it was me then I would request a copy of my police record. If the incident does not appear at all in the polcie records then it probably means the police did not make a report of the incident at the time and it is a nothing. You are probably safe answering No even if you were over the age of 18 if that is the case.

Probably not as it's not a conviction. Check the US visa guidelines to be sure.

It is likely it was just a misdemeanor, where that very well may have something to do about it, anymore. So a Formal letter upon those charges should be submitted by you, for getting you the visa. Misdemeanors used to be ignored, but that is just not the case anymore.

Yes, do not come here.

Probably. Look for the USA website for visa guidelines