Context :

Aims : We extend the gr -band time coverage of the gravitationally lensed double quasar Q0957+561 .
New gr light curves permit us to detect significant intrinsic fluctuations , to determine new time delays , and thus to gain perspective on the mechanism of intrinsic variability in Q0957+561 .

Methods : We use new optical frames of Q0957+561 in the g and r passbands from January 2005 to July 2007 .
These frames are part of an ongoing long-term monitoring with the Liverpool robotic telescope .
We also introduce two photometric pipelines that are applied to the new gr frames of Q0957+561 .
The transformation pipeline incorporates zero-point , colour , and inhomogeneity corrections to the instrumental magnitudes , so final photometry to the 1-2 % level is achieved for both quasar components .
The two-colour final records are then used to measure time delays .

Results : The gr light curves of Q0957+561 show several prominent events and gradients , and some of them ( in the g band ) lead to a time delay between components \Delta t _ { BA } = 417 \pm 2 d ( 1 \sigma ) .
We do not find evidence of extrinsic variability in the light curves of Q0957+561 .
We also explore the possibility of a delay between a large event in the g band and the corresponding event in the r band .
The gr cross-correlation reveals a time lag \Delta t _ { rg } = 4.0 \pm 2.0 d ( 1 \sigma ; the g -band event is leading ) that confirms a previous claim of the existence of a delay between the g and r band in this lensed quasar .

Conclusions : The time delays ( between quasar components and between optical bands ) from the new records and previous ones in similar bands indicate that most observed variations in Q0957+561 ( amplitudes of \sim 100 mmag and timescales of \sim 100 d ) are very probably due to reverberation within the gas disc around the supermassive black hole .