Ah, sorry. Wasn't thinking properly before. You'd have sent it all off to the clerk of the court that's dealing with it right? Presumably if they're really camera happy round there your form could still be halfway down the pile of details sent by all the other drivers they've zapped recently.
When I got my last NIP some years ago I asked for the photo and it hadn't arrived by the time the 28 days was up. I rang 'em up and was told not to worry, as they would send a reminder out if they didn't get the form in time. Sure enough I got a nagging letter not long after saying they'd give me another week. (Bloody photo never appeared though, I gave up on that in the end).
Two caveats about that. For all I know the not-so-gentle reminder I got might be unique to Thames Valley. Plus it was about five years ago, pre-pratnership days anyway, and they've not exactly got more forgiving in the meantime have they?
Maybe it's better to chase them sooner rather than later. The form could be under a chair in a sorting office somewhere, and I wouldn't trust them not to blame you for not sending it recorded or paying by phone.