Have you ruled out Tabanus for any reason? That was my first thought but I'm no fly expert. However, the legs look more like a Tabanus to me. See -
I think you are right,the tapering body made me think of Thereva but other features certainly favour Tabanus .It may be a male ,they have generally slimmer bodies ,unless theyv'e been on the ale.
Excellent. That definitely looks like it. I'm amazed we get flies that big - my 1 1/2 inches was a conservative estimate!). Thanks both for taking the time to reply.