  We use a sample of 17 strong gravitational lens systems from the BELLS
GALLERY survey to quantify the amount of low-mass dark matter haloes within the
lensing galaxies and along their lines of sight, and to constrain the
properties of dark matter. Based on a detection criterion of 10$\sigma$, we
report no significant detection in any of the lenses. Using the sensitivity
function at the 10-$\sigma$ level, we have calculated the predicted number of
detectable cold dark matter (CDM) line-of-sight haloes to be $\mu_{l} =
1.17\pm1.08$, in agreement with our null detection. Assuming a detection
sensitivity that improved to the level implied by a 5-$\sigma$ threshold, the
expected number of detectable line-of-sight haloes rises to $\mu_l =
9.0\pm3.0$. Whilst the current data find zero detections at this sensitivity
level (which has a probability of P$^{{\rm5}\sigma}_{{\rm CDM}}(n_{\rm
det}=0)$=0.0001 and would be in strong tension with the CDM framework), we find
that such a low detection threshold leads to many spurious detections and
non-detections and therefore the current lack of detections is unreliable and
requires data with improved sensitivity. Combining this sample with a subsample
of 11 SLACS lenses, we constrain the half-mode mass to be $\log$(M$_{\rm hm}) <
12.26$ at the 2-$\sigma$ level. The latter is consistent with resonantly
produced sterile neutrino masses m$_{\rm s} < 0.8$ keV at any value of the
lepton asymmetry at the 2-$\sigma$ level.
