Its just pure luck Nicola! I actually went with Egret shots in mind. Pauline has managed amazing shots of them from The Spinnies. I turn up and have the same problem as you, the damn birds are half a mile away!
So sat for an hour or two infront of a tree near a couple of feeders and the Nuthatch appeared! I'm sure, in the same spot, you'd get just as good images with the bigma!
I've got my head around the fact now, I can aim to go out with an image or species in mind but also be flexible!
Keep shooting, those shots will come!
good luck.
Stu