Hi
Its not the speed, its the revs you need, the DPF is like a filter, thats blocks up and the only way to clear it is heat, higher than normal revs get the dpf hotter and allow to burn the soot off
at 70mph in my S-type R in sixth gear would barely be doing just over a 1000rpm
select a lower gear and keep the revs up a bit more, for 15 mins or so just to get the dpf up to temp
cheers
Joe