From the HMRC website:

Q10. What happens if I let my UK home/property whilst I am living or working abroad?

A10. Letting agents, or certain tenants if there is no letting agent, must deduct basic rate income tax from the UK rental income if the landlord has a usual place of abode outside the UK. However, landlords have the option of applying for approval to receive their UK rental income with no income tax deducted by simply completing form NRL1 (PDF 205K).

The granting of approval does not grant exemption from UK income tax; any tax liability will be dealt with under Self-Assessment.

If the rental income is also charged to tax in your country of residence, then that country should give the relevant Tax Credit for the UK tax paid.

If you are not in Self-Assessment, you have paid tax and want to claim a repayment, please complete and send us form R43.

If your rent is received gross and you did not receive a Self-Assessment return, the tax return is not necessary if no tax liability arises.

If you jointly own the property, the income is usually shared equally; owners are individually responsible for any UK tax liability arising from their share of that income.
Therefore, each owner has to complete form NRL1 (PDF 205K), make a claim for repayment and complete a tax return in respect of your own share.

Further information about the Non-Resident Landlord Scheme is available.

[source: http://www.hmrc.gov.uk/cnr/faqs_general.htm#10nr ]

Also check out non-resident landlord's scheme information:

http://www.hmrc.gov.uk/cnr/nr_landlords.htm