I think I have found the right part now. I think this is the OEM part but not branded Volvo.
https://cat.hella.com/web/public/hella/en/100000/$catalogue/2/$product/6PX%2B012%2B684-061_5750/datasheet.xhtml;jsessionid=495406B82321241ECF7856B 1851BD1B5
I bought one here
http://www.buycarparts.co.uk/hella/7915643
Just over 1/2 the Volvo price. I'll let you know if it works OK or not.
Hope that helps.
Greg