You can ask for anything you like Matt, but getting it is another thing...I go through it with my wife every night...
David, personally, I think that, if they dig their heels in, you've got a snowflake's chance in hell of getting a late fee paid. Legally, I doubt you have a leg to stand on, unless 1) you get Consumer Affairs involved and/or 2) can demonstrate some kind of financial loss as a result of not having the car for so long.
I can't work out what you asked them to do exactly and who was covering the bill - did you pay them to respray the car in addition to the work they had to do for the insurance? If it was a straight insurance job, I can almost guarantee that they're making virtually no money off the job, which means they would've pushed it to the back of the shed until they weren't busy.
Next question - why did they have to replace the battery? I have a few more yet, but I'll get to them later. I'm just struggling to get my head around it all. (past my bedtime!)