Cemex Buys 18.8 Percent Stake in RMC  LONDON (Reuters) - Mexican cement company Cemex said on  Monday it had spent about 432 million pounds (\$780 million)  buying an 18.8 percent stake in RMC Group &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=RMC.L target=/stocks/quickinfo/fullquote"&gt;RMC.L&lt;/A&gt; as part of  its planned takeover of its British rival.