There are 2 methods the tax man will accept, the first is a pecentage of your Electric/Gas/Water and council tax bill, to go down this route you need to use a "room" solely for business, so if you use a spare bedroom, there must NOT be any bedroom furniture in it.
To work out the percentage you count how many rooms you have in your house, you cannot count the kitchen, hall, gargage, toilet or bathroom.
you then divide the bills into how many rooms you have and divide down to the 1 you are using.
The other method is far simpler, you can claim a weekly usage rate, the only problem with this is each tax office has its own figure on what it classes as a reasonable weekly rate, its best to phone them and ask, I though claim £5 per week = £260 per year.
also don't get fooled into claiming anything you think is right, because the taxman will accept everything you put down on your tax return, the problems start is when they look at YOUR return in detail and you then have to convince them that, the expenses you are claiming for are right and proper.
The majority of fines that are appied to tax payers are the results of wrong information being supplier or claimed for.