Hi Moira, Mike is absolutely correct about the light ( or lack of it ), this lens will take some time to get used to.
At it's quickest it's f5, so its not the fastest lens out there , but never the less it is a good lens for the money, so keep trying.
As we are coming into winter the light is going to be even less. I did notice yesterday on your Exif that you were using shutter priority which I think is fine, although most would probably say use aperture priority, which is fine if you have the light required, may I suggest using a slower speed than 1/640th sec, if the bird like the blue tit is relatively still, 1/300th sec maybe, this should help ( if you can hold this lump still of course )
I have this lens so I can sympathise with your frustrations but, do give yourself time, and keep experimenting.
As for your wagtail, no, you dont see many, so very well done ..............Rob