I'm quite clear now that my legs are a problem for me. When using a pull buoy I feel faster (not sure if I am) and less tired than using my legs. Lengths of with just leg kicking are glacially slow and barely achievable. Basically my legs are wasting energy and probably slowing me down.
Having scanned the swim-smooth site once again a contributory factor could be over lifting my head when breathing as they comment that this pushes the legs down. With a pull buoy this will be resisted and help efficiency. At the Goodwin I tend to rotate a lot for breathing as the pool is so choppy.