Sorry no . I really don't want to buy a return ferry ticket . It has been suggested you are supposed to though I can't see any evidence of that .
When you said your passport had been stamped for your return date I wasn't sure if that was your ferry date or your official 90 day limit
Passport is stamped with your date of exit from the U.K. You may travel in and out if the Schengen zone, and your passport should be stamped at every external border, in and out, so the original stamper wouldn’t know your return date.