Time limit for Tax Return

Doing the Tax Return for a client and it transpired when they moved during 2010/11 they kept the old house and now rent it out.

The wife has not been asked for a tax return as HMRC are ignorant of the income.

Obviously she will need to return this income but what are the time limits: do we write now and tell them, within the 2010/11 time limit, and then get 3 months from when they request a return, or do we have to try and get a UTR and file the form by 31st January?

Thanks in advance for your help.