  We analyse a group of radio sources, a subset of the 200 mJy sample, all of
which have core-jet radio structures measured with VLBI and have flat spectra
stretching from the radio to the millimetre/sub-millimetre band. Thus the
objects have most of the properties expected of blazars. However, they display
varied optical properties ranging from ``Seyfert-like'' objects, through BL Lac
objects, to ``normal'' elliptical galaxies. We investigate the distribution of
synchrotron peak frequencies in their Spectral Energy Distributions (SEDs) and
find a broad distribution between 10^12 and 10^16 Hz. Our conclusion is that we
should consider virtually all objects in the sample as blazars since much of
the diversity in their classification based on traditional optical criteria
arises from differences in the frequency at which the non-thermal emission
begins to decline. Specifically, an object is only classified as BL Lac when
its peak frequency falls in the near IR/optical range.
  We determine peak frequencies using the same method for objects from other
blazar samples. An important result is that our objects do not follow the
blazar spectral sequence proposed by Fossati et al. and Ghisellini et al. in
which, on average, peak frequencies increase as the radio luminosity decreases.
Most of our low radio-luminosity sources have peaks in their SEDs at low
frequencies, not at the expected high frequencies. We suggest that at least
part of the systematic trend seen by Fossati et al. and Ghisellini et al.
results from selection effects.
