To squash or not ....

There are four of these scary big buzzy things terrorising me in the greenhouse. I'm trying really hard not to kill them but they're not making it easy for me. This one was preventing me from getting to the rocket and parsley for tonights dinner!
I assume they are horseflies but as they've not attempted to bit like all the other smaller ones I'm curious. 
Thanks to osuzanna for hosting Tiny Tuesday this month.