In relation to CGT is does not matter if the property is let or not.
Once the property is no longer your PPR you have one year to dispose of it before CGT is applied. The gain is calculated from the time you bought the house and is calc on the percentage of time that it is not your PPR less 12 months.
The letting of the property is liable to income tax and is calc as follows:
Rental Income ..................... 12,000
Less
Interest Payments ...5,000
Wear & Tear ...........2,000
Other ....................1,000
............................................8,000
Profit ....................................4,000
This is a very simplified version
This will be added to your other income and you will be taxed accordingly
If you currently pay tax at 41% you have to pay 1,640 in tax
For more info on Rental Profit see [broken link removed]