Freelancer Expenses and Tax reduction

So, had to purchase a new IMac this year for my freelance work.

Would be interested to know what value of the product I am able to expense.
I use it for work and for homelife too, so have read I can claim 50% off my tax assessment.

To add to this, is there a perfect resource that provides us with a list of all the things we can expense, as I feel the HMRC is quite vague on these answers. For example, how much can you claim against living costs etc..

Any help, greatly appreciated.