Very difficult to say exactly without knowing something of the switching topologies used but the price bracket indicates triac choppers generating all kinds of harmonics and horrible noise on the line with low grade output filters in the soundlabs.
The switching supplies generating your LV are designed (at the outset) to work on full line voltage and as such the input filters on the switching section is designed for that (bare minimum) senario, when the line waveform is chopped by the dimmer pack the input filters do not store enough energy to holdup the input supply to the switching section of the transformer and discontinuous operation in the output filters is the result. This is because the inductors and capacitors in the filters are probably the most expensive components in the whole assembly and are value engineered to the knuckle. The fact that the whole thing seems to be dimmable is just a (un)happy accident masked by the thermal inertia of a regular tungsten lamp.
You can try different combinations of electronic transformer - price is as Andy has said before irrelevant, or a copper and iron 50Hz transformer, that will stop the flicker but will be phsically noisy at low dim and need to be oversized to cope with the thermal problems associated with the dimmer harmonics.
NJD have always been technically excellant with some very cunning designs (look at an early IQ250 waggly mirror!!) but they are built to a strict price (not quite the lowest) Richard is probably seeing the results of careful optimisation of where the money goes.