  We report on the multi-frequency timing observations of 21 pulsars discovered
in the LOFAR Tied-Array All-Sky Survey (LOTAAS). The timing data were taken at
central frequencies of 149 MHz (LOFAR) as well as 334 and 1532 MHz (Lovell
Telecope). The sample of pulsars includes 20 isolated pulsars and the first
binary pulsar discovered by the survey, PSR J1658$+$3630. We modelled the
timing properties of the pulsars, which showed that they have, on average,
larger characteristic ages. We present the pulse profiles of the pulsars across
the three observing bands, where PSR J1643$+$1338 showed profile evolution that
appears not to be well-described by the radius-to-frequency-mapping model.
Furthermore, we modelled the spectra of the pulsars across the same observing
bands, using a simple power law, and found an average spectral index of $-1.9
\pm 0.5$. Amongst the pulsars studied here, PSR J1657$+$3304 showed large flux
density variations of a factor of 10 over 300 days, as well as mode changing
and nulling on timescales of a few minutes. We modelled the rotational and
orbital properties of PSR J1658$+$3630, which has a spin period of 33 ms in a
binary orbit of 3.0 days with a companion of minimum mass of 0.87$M_{\odot}$,
likely a Carbon-Oxygen or Oxygen-Neon-Magnesium type white dwarf. PSR
J1658$+$3630 has a dispersion measure of 3.0 pc cm$^{-3}$, making it possibly
one of the closest binary pulsars known.
