  We theoretically characterize the collective radiative behaviour of N
classical emitters near an interface between different dielectrics that
supports the transfer of surface plasmon modes into the far-field of
electromagnetic radiation. The phenomena of superradiance and surface plasmons
can be combined to amplify the emitted radiation intensity S as S= A N^2 S_0
compared to a single emitter's intensity S_0 in free space. For a practical
case study within the paper A=240, compared to A=1 in free space. We
furthermore demonstrate that there are collective modes for which the intensity
of the emitted radiation is suppressed by two orders of magnitude despite their
supperadiant emission characteristics. A method to control the emission
characteristics of the system and to switch from super- to sub-radiant
behaviour with a suitably detuned external driving field is devised.
