Just found this on E-Bay.
It's one of the 'Approved Options' for the Series 1 - a Brockhouse trailer.
They were sold alongside the Series 1 through the Rover dealers.
This one is the correct size & has the original body panels that bear the pressed crosses for strength.
Anyone just finished a Series 1 re-build & fancies the finishing touch??
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=9882&item=2491072059&rd=1