the tax man will work out what percentage of your time you spend working at home for you business, then he will allocate that percentage to your bill ie if he thinks you spend 5 percent of your time working at home he will allocate 5 % of your bills to it,
with the water i write down how many lit of water i use daily ie 250, 350 etc then i invoice my company for there share of the water bill that way the compayy pays for what water it uses
is the same with my van and car, i got a van that i claim everything on and i dont claim anything on the car, even though i might use it, i also pay the insurance out of different bank accounts, so that i keep both account and bills separate
the problem i see with tax return time, is everyone has that mate who earns loads and hardle pays any tax, yet never gives you the no of his accountant lol