In reply to druss:
Yes, this is what you do. Report the 'gross' income - in your case £6000 in the income box of the property pages of your tax return.
Then complete the boxes for expenses.
OR use the £1000 property allowance, so net taxable income is £5k.
You can't do both. As in your can't claim the agency charge AND the property allowance.
So add up agency fee plus any other costs e.g. annual gas certificate, buildings insurance, repairs you made to the property, mileage at 45p per mile if you needed to drive to the property to inspect it, etc.
If that is more than £1k then claim those expenses for a lower taxable profit.
I'm a qualified accountant and run my own Accountancy firm