TO TRADE WITH THE EU DO WE NEED A TRADE AGREEMENT?

No we don't!  The US, Russia, and China are the biggest traders with the EU.  In 2014, China exported to the EU 302 billion dollars worth of goods without a trade agreement.  The UK exported 191 billion dollars. (See page 202 of Inconvenient Truths About UK Trade and the EU, by William Dartmouth).  

Of course, the UK, US, Russia, China, and all the other countries of the world don't sell anything. Only individual businesses trade with each other.

As a consequence of EU membership, the UK cannot enter into a Trade Agreement with another country.  When Cameron goes on a "trade delegation" his hands are tied behind his back by the EU.  He can merely promote British sales- his skills in PR really come in.  Any sales must be within EU rules.