and how many have their been in, say, the last 6 years.

Definitions:

a) "genuinely new independent label" as in a label founded by someone with no association with existing successful bands or labels and signing bands that were not previously well-known.

b) "genuinely successful" in that it has at least 3 acts who would be known to casual music fans (i.e. be played regularly on XFM or Radio 1, get decent billing on festivals, have top 10 or at least top 20 albums etc.)?