Graphics, Figures & Tables ⇒ Multirow table problem

[yago]

Hello to all. I have a problem with a table with multirows and I can't to center the headers. Of course in the preamble I have \usepackage{multirow}. This is the LaTeX code for the table:

\renewcommand{\arraystretch}{1.2}
\begin{center}
\begin{tabular}{|>{\centering}m{2cm}|>{\centering}m{1cm}|>{\centering}m{1cm}|>{\centering}m{1cm}|>{\centering}m{1cm}|>{\centering}m{1cm}|>{\centering}m{1cm}|}
\hline
\multirow{2}{2cm}{\centering{Distancia en mm. del centro de la esfera al de la rosa}} & \vspace{10pt}{\multicolumn{6}{c}{Diámetro de las esferas en mm.}}\tabularnewline
\cline{2-7}\\[20pt]
& 101,5 & 127 & 152,5 & 178 & 216 & 254\tabularnewline
\hline
317,5 & $0\gs 30'$ & $1\gs 10'$ & $2\gs 30'$ & $3\gs 50'$ & $6\gs 10'$ & $10\gs 05'$\tabularnewline
\hline
311 & $0 35$ & $1 20$ & $2 50$ & $4 10$ & $6 55$ & $11 25$\tabularnewline
\hline
\end{tabular}
\end{center}

I need a table like the attached file. Thank you very much for your help