I rent out my house and my mortgage interest is bigger than my rental income. Can I claim tax relief on this loss? And do I need to tell HMRC?
Yes, you can claim tax relief on a loss. But only against other net rental income - either in the same year or carried forward to future years.
I would advise you to notify HMRC that you are renting out your property by requesting a Tax Return. It will then be the case that any net losses you make are on the record, and can get future tax relief.
There is a web page that gives guidance on who needs to complete a tax return: http://www.hmrc.gov.uk/sa/need-tax-return.htm