snoogle,
I believe these are called "Square bonds" and are becoming the norm in the FW world. A colleague of mine has recently been 'pooled' for 'flybe' with a very similar bonding system.
As to the legality - I know two folks who bailed out on bonds - one assures me he got away with it scot free - the other one was right royally stuffed.
I guess you signs your bond and takes your chance.
Given the figures you've quoted this must be for more than just an EC type rating - what's the rest of the story!
R1