If you sold shares in May 2011 you aren't due to file the return for this disposal until 31/10/12.
When you say you submitted a return form, what do you mean? I doubt the 2011 CG1 paper return forms weren't in circulation by February, so perhaps you've filed a CG1 that was processed as a 2010 return? (In which case the assessing software would believe it is late and surchargeable.)
Your first step should be to check what period the assessment / balancing statement you've received relates to. If it says 2010, then they've assessed you for the wrong period, and it'll be easily fixed.
If it is a 2011 assessment then it's very strange, because it should'nt really be possible to surcharge 2011 yet! But again it should be easily fixed by speaking to a person in your local tax office who deals with CGT...