  We present new observations of the gravitational lens system CLASS B0128+437.
HST observations detect a very faint, extended object in I-band with no
emission from the lensed images visible; no detection at all is made in V-band.
The lens system is detected with much higher signal to noise with UKIRT in
K-band, but the resolution is not sufficient to allow the lensed images and the
lens galaxy to be separated. A careful astrometric calibration, however,
suggests that the peak of the infrared emission corresponds to the two merging
images A and B and therefore that the lensed images dominate at infrared
wavelengths. The new radio data consist of VLBI radio images at three
frequencies, 2.3, 5 and 8.4GHz, made with the VLBA and the 100-m Effelsberg
telescope. The lensed source consists of three well-defined sub-components
embedded in a more extended jet. Due to the fact that the sub-components have
different spectral indices it is possible to determine which part of each image
corresponds to the same source sub-component. Our main finding is that one of
the images, B, looks very different to the others, there being no obvious
division into separate sub-components and the image being apparently both
broader and smoother. This is a consequence we believe of scatter-broadening in
the ISM of the lensing galaxy. The large number of multiply-imaged source
sub-components also provide an abundance of modelling constraints and we have
attempted to fit an SIE+external shear model to the data, as well as utilising
the novel method of Evans & Witt. It proves difficult in both cases, however,
to obtain a satisfactory fit which strongly suggests the presence of
sub-structure in the mass distribution of the lensing galaxy, perhaps of the
kind that is predicted by CDM theories of structure formation.
