UPDATE - Blizzard have now officially announced the release date on Battle.net and pre-purchases are open.
Eurogamer has reported that Heart of the Swarm well be released on March 12th, 2013. The information comes from Eurogamer's Spanish site, who spotted a listing on Battle.net. An official announcement is expected later today.
It's uncertain at this time if the March 12th date is for a worldwide release or merely North America. What is known is that Blizzard will be selling a Wings of Liberty and Heart of the Swarm double pack for £45. The new game alone is listed at €39.99 in Europe, meaning it will likely sell at £30-35 over here.