Sale, Harlequins, Exeter and Saracens have qualified for the Premiership Rugby Cup semi-finals.
Simon Hammersley and Akker Van Der Merwe also went over for the Sharks, with Du Preez adding seven points to nullify the effect of home scores from Saia Fainga’a, Theo Brophy-Clews and Ben Loader.
Paddy Jackson matched Du Preez’s haul with the boot, ensuring a losing bonus point for London Irish.
Ben Spencer ran in two tries and kicked 13 points as Saracens defeated Pool B winners Harlequins 28-21 to reach the semis as the strongest runners-up at the end of the group stage.
Rotimi Segun got over first for Sarries before Spencer took over point-scoring duties, contributing 23.
Marcus Smith kept Quins afloat at Allianz Park with three penalties and a conversion of Mike Brown’s try, while Gabriel Ibitoye recorded their late consolation try.
Jonny Hill, Jack Maunder, Sam Simmonds and Harry Williams all dotted down before half-time and Dave Dennis did so on the hour mark, with the Simmonds brothers converting every try of the match except Marcus Street’s late contribution, for which Gareth Steenson obliged.
Their poor kicking came back to haunt them though as Dingwall dazzled and James Grayson’s flawless kicking bolstered a 31-22 Northampton victory. Both outfits missed out on a last-four place, however.
Qualifying had looked like mission impossible for Wasps but Tom Cruse crossed after four minutes before Watson led a second-half onslaught against the Warriors, with Josh Bassett also crossing late on for the hosts.
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