About 15 minutes after posting to say I didn't have any problems, the interactive service disappeared for about half an hour. When it came back, all four feeds were showing the same pictures (same as the main channel) although they were still labeled differently. Since then, things seem to have gone back to normal.
Shame that they didn't think to have an on-screen graphic or something advising of the problem. But after being conned into paying for a non-interactive Formula One weeked a while back, I've learned that Sky do not like to acknowledge their problems. At least E4 is not PPV and the problems arose when they were still showing live coverage on the main feed anyway.