The first time your Cateye light makes a bid for freedom from its bracket, that should be taken as a cue to replace the bracket (a mere £4.50 from Evans).
The second time it happens, this is a warning that you really should do something about it very soon.
The third time it happens... damn.
Fortunately, it's just the battery cartridge casing that's broken, not the light itself, and the battery is - in theory - replaceable. Only it's an older model (Volt 700) and I have so far had no luck finding anything on the interwebs. I would try to repair the battery but the wires have become disconnected and I don't think it would be such a good idea to wield a soldering iron near a Li-ion battery.
The real shame is that it's an excellent light that has served me well for over four years. The Volt 700 was superseded by the Volt 800 which aiui is identical except for being the one with the bigger lumens, and I would happily get a new one... if only I could afford to spaff away £90 for no good reason (wiggle seems to have them for £57 but that's still more than I've got in the petty cash tin at the moment).
Also note: at this time of year, it's a good idea not to leave your bike outside overnight as the rear derailleur may freeze solid, leaving you stuck in your lowest gear for the mad dash to the station. Also, when you eventually get to the station (in time to see your train pulling away) you may find that your lock is frozen too and it takes a good ten minutes to defrost it enough to get the key in.