If you are thinking of the Helton (and still reading this thread)
<p dir="ltr" style="margin-top:0; margin-bottom:0;">Used tandem bike carrier for roof rack</p> <p dir="ltr" style="margin-top:0; margin-bottom:0;">Slides up for easier mounting</p> <p dir="ltr" style="margin-top:0; margin-bottom:0;">Will link website and YouTube for further info</p>
www.ebay.co.uk
I doubt it will sell that that £179, but you could make an offer.
As saneagle says, the increased fuel consumption will be massive. (That was another reason we gave up the idea of putting our tandem on our roof, which I'd forgotten to mention in earlier post. Especially as it would have been on the car for the return journey from the South of England to North West Scotland. I had mentioned rain.)