You can travel with a passport on the 10th March 2011 if you wish. As long as it's valid it doesn't matter. If your going back over to spain before the 11th of March and decided you wish to come back after the 11th of March 2011 then 'yes' I'd personally get it renewed.
EU law requires all EU member countries to recognize the validity of any EU passport (including those of the UK) right up until the final second of its stated validity. So (as long as you hold an EU passport and you're travelling solely within the EU), you can continue to use it right up until 11/03/11.
<<If you were going outside of Europe you have to have six months left on it>>
Even that's not 100% true - there are countries to which, yes, it applies, but there some countries that require only 3 months, and yet others that have no minimum requirement at all.
And with some the answer varies according to who you ask and how you phrase the question......