alter master page HTML in codebehind of a user control

I have a master page with the following tag

<section id="bannerSection">

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I'd like to put a user control on some of these master pages which alter this HTML to:

<section id="bannerSection"  style="background: url(Libraries/banners/slide1.jpg) no-repeat center center; ">

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I presume the first thing i need to do is put run at server on the html control on the master page

<section id="bannerSection" runat="server">

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but what code do i need to put in the Usercontrol to search for this div and alter it with the style...
?????????

Its a bit more involved with a CMS and different bg images etc, so i've stripped the question back to its basic form
but if i can do exactly what i've said above, i can figure the rest out myself so it works in the CMS