I am really lucky - I have been with two mobile company's who folded.
Easymobile folded, and strangely changed their name to something else first - you had about two months warning - but got to port out.
Blyk folded too, and there was about a month to port out.
I think both said you would not get credit back initially - so you would be best using it - then in the last few days said you were entitled to reclaim it.
I lost my number on Easymobile, as I was not in the UK over the time it happened, and so had no easy way of getting something to port to.
I wonder how long until fat-boy's airline goes the same way?