Question becomes sign-ups on what basis? Was it Miliband who changed the rules to allow anybody to buy a Labour leadership election vote for £3 ...
Or "register of interest" where virtually all reporters from every local rag together with pollsters, anybody with any political interest ... all wanting to find out what's going on and without any real support.
Or maybe genuine left leaning current and ex-Labour members. difficult to tell but lots of possibilities (if you are as cynical about politicians as I've become).
Ian