Thanks Roger and everyone, will have to get the step ladder out in the morning then, as the rape is too high for me to see above this year. Great shame, as I was hoping to get some Grey wagtail action shots this spring with the big lens, only had the 150mm last year and everything was too far away, but of course a 500mm with teleconverter takes a lot of holding, not easy when perched on a step ladder.
Really shows the value of forums like this, I've now got and ID for the mistery birds and also been given an idea of how to catch a glimpse of them.