Clearing iPhone mail storage from iPhone 6S

Despite clearing almost all mail messages from Mail the Documents & Data remains at 3.25GB. I have restarted the phone by switching it off but makes no difference.

The phone syncs with Gmail and home computers. The computers and the iPhone show that the messages are deleted with only around 30 emails in the Inbox and 20 in Sent.

No files in the Bin or Trash yet 120 messages in All Mail.

What is the difference between the Bin & Trash folders?

Driving me crazy.

Regards

Colin