I think it means that the absolute minimum he must serve is 5 years 7 months, if he does everything right, forfills every possible criteria for release, and is deamed no longer a risk, but the "indefinate" part of the sentance means that if they wish he can be held for just that, an indefinate period of time (Life has an upper limit).
My guess is the judge cannot sentance him indefinately without the option of release, but by doing it this way the guy can be locked up for life (as in life, not 20 years, out in ten).
I think there used to the option to have someone held without the minimum time in which they could be released (basically at the Home secretaries pleasure), but it was ruled illegal under EU law or something, so this is a workaround - the option for release is there, but it's not likely to be excercised.