This was a young one I think
I thought it might be a grey wagtail but they are yellow that makes sense doesnt it
https://www.rspb.org.uk/birds-and-wildlife/wildlife-guides/bird-a-z/grey-wagtail
It most resembles the cape wagtail as you said
Hmm wonder whats it doing here? And what the wells's in its other name refers to gonna have to do some more research