How likely am I to get my deposit back?

I think you should tell the truth but say that whilst you realise that a reasonable deduction for administration costs is entirely appropriate, you would be most grateful if ... blah, blah, blah. If it took you a bit of time to actually sort out what you are going to do, just admit to that and throw yourself on their mercy. I think you are more likely to get it back that way - they will see the lies as a personal insult to the intelligence and a challenge and be against you.