After much aggro I've sorted the issue. My iPhone was showing 50 mails unread but when I went to inbox or separate Unread there was no trace of the 50. I tried the various solutions offered on here but finally one thing worked:
I deleted the account in 'Settings'.
Then added the same account I'd just deleted.
When first reopened it downloaded 95 new emails (all of which I'd previously deleted but clearly there was a trace left of them somewhere albeit goodness knows where!)
I then pressed 'Edit' and one-by-one checked on each email.
Eventually after checking on all 95 I pressed 'Trash"
They went and so did the 50 tag on my email icon that had been bugging me and frustrating me for hours.
It's a long haul but i hope it works for anyone suffering this issue.
Aside, i used to have a Blackberry. You could delete all simply by highlighting them all in one foul swoop. People used to tell me BB for business users, Apple for people who like playing with toys. Now I understand what they meant!
Come on Apple. Not being able to delete 100's emails in one go is ridiculous. Sort it please. I'm sure there are many like me who get lots and lots of emails.