The E6 process is extremely sensitive to contamination of the first developer with the second (colour) developer. Even vapour contamination may cause a loss of density and a noticable colour cast; which sounds very much like your description of the problem.
Equally, contamiantion of either with the bleach solution may cause simailr results.
I would take the matter up with Jessops, although I'm afraid there's nothing much that can be done with your film now.