Tom Youngs, Tom Croft and Geoff Parling are among the players released back to their clubs from England training ahead of this weekend's Aviva Premiership Rugby fixtures.England retain 16 players for the remainder of the training week at Pennyhill Park, including Ben Youngs and Dan Cole who have started all three matches so far in the RBS 6 Nations.   Hooker Youngs and back-row Croft have been selected among the replacements in the three rounds so far and will now be available for Tigers as they prepare for the trip to Newcastle Falcons.   Second-rower Parling, who returned from injury to play the full 80 minutes in Tigers’ win over Sale Sharks at Welford Road last Saturday, is also available for club duty again this week.   Ed Slater and Manu Tuilagi will be in camp with England this week as they continue their rehabilitation from long-term injury.   Players retained for England training: Dave Attwood (Bath Rugby)  Mike Brown (Harlequins)  Luther Burrell (Northampton Saints)  Dan Cole (Leicester Tigers)  George Ford (Bath Rugby)  Dylan Hartley (Northampton Saints)  James Haskell (Wasps)  Jonathan Joseph (Bath Rugby)  George Kruis (Saracens)  Joe Marler (Harlequins)  Jack Nowell (Exeter Chiefs)  Chris Robshaw (Harlequins)  Billy Twelvetrees (Gloucester Rugby)  Billy Vunipola (Saracens)     Anthony Watson (Bath Rugby)  Ben Youngs (Leicester Tigers)