I am seriously considering a good prime telephoto and awaiting the release of the Pentax 560mm with interest. If that doesn't pan out then I may consider the Sigma 500mm, from which I have seen some stunning images.
I also want to try to get some video of the Kingfishers so will definitely be taking the tripod with me next time. One other option I am keen to try, which I have used before, is my 80mm ED astroscope. It has the reach, with a 600mm focal length, and can produce some very detailed shots with a boost of contrast in PP. Might give that a go next week as I think we have some good weather coming.
This was a test shot of a Dunnock from a while back at probably about the same distance from me as the Kingfisher. Interesting to use as fixed aperture and manual focus, but worth a shot, if only to exclude it from any further bird photos.