Go to this AA site...
http://tinyurl.com/382vzg
where it says...
Regeneration is either passive or active
Passive regeneration
Passive regeneration takes place automatically on motorway or fast A-road runs when the exhaust temperature is high. Because many cars don't get this sort of use vehicle manufacturers have had to design-in 'active' regeneration where the engine management computer (ECU) takes control of the process.
Active regeneration
When the soot loading in the filter reaches a set limit (about 45%) the vehicle's ECU will initiate post combustion fuel injection to increase the exhaust temperature and trigger regeneration. If the journey is too short while the regeneration is in progress, it may not complete and the warning light will come on to show that the filter is partially blocked.
It should be possible to complete a regeneration cycle and clear the warning light by driving for 10 minutes or so at speeds greater than 40mph.
Good Luck..., February 2016