I have just searched for hoverflies and found the largest hoverfly in the britain to be the Volucella zonaria hornet mimic hoverfly.
It looks similar so I guess that's what it is.
I actuall thought it was a Hornet at first sight as well.
Thanks for the help.
Nice bit of detection by you too. Volucella zonaria is indeed a large hoverfly with a wingspan of around 45mm and it does do a great impression of a hornet. Hoverflies are true flies rather than bees or wasps, and are harmless, even though this one does look quite alarming!