Thanks Arun, I had to modify it slightly but your answer got me there. Thank-you!
<item>
<guid isPermaLink="false"><%# Eval("SKUGUID") %></guid>
<g:id><%# Eval("SKUName") %></g:id>
<title><%# Eval("SKUName") %></title>
<g:description><%# Eval("SKUShortDescription") %></g:description>
<g:link><%# GetAbsoluteUrl(GetProductUrlForFeed(Eval("SKUGUID"),Eval("SKUName"),Eval<int>("NodeSiteID")),Eval<int>("NodeSiteID")) %></g:link>
<g:image_link>http://185.26.149.74/webimages/<%# Eval("SKUName") %>.png</g:image_link>
<g:availability>in stock</g:availability>
<g:price><%# Eval("SKUPrice") %> GBP</g:price>
<g:brand><%# Eval("SKU.SKUManufacturer.ManufacturerDisplayName", true) %></g:brand>
<g:mpn><%# Eval("SKUName") %></g:mpn>
</item>