I have received a letter from my tax office saying that they intend to make an enquiry into my tax return. What does this mean?

For 2006/07 self assessment tax returns H M Revenue & Customs can enquire into the return within 12 months from 31 January 2008 (or if the return was issued late, within 12 months of the filing date).
From 2007/08 onwards H M Revenue & Customs can only enquire into the self assessment tax return up to 12 months after the date they received it, e.g. the enquiry window for a return filed on 1st May 2008 closes on 30 April 2009.
The enquiry may be into a specific item shown on the return or just a general query. You should always respond within 30 days and seek professional advice where you are unsure.