good posts tosh.
The cast iron defense has nothing to do with accountants, it's that you have another vehicle, a car.
That's an absolute defence.
Of course if you don't have another vehicle then a proportion may be for private use.
Assuming you are at the early stages of an enquiry this is probably a ploy to give you something to think about and will probably be dropped once they get their teeth into your cash situation.
The best strategy is a slow retreat, you have to let them win, but be carefull because they go back to when you started your current set up or seven years, so £500 soon becomes £3000. Be cooperative, just this side of groveling, although they don't object tp groveling.This will avoid penalties.
If you really are up the creek i can give you a couple of off forum survival tipps.