My guess is that they're co-sleeping with the baby in a sleeping bag to keep them warm. That in and of itself seems like quite a big risk from a SIDS point of view.
They could also easily make a mistake while trying to keep warm and induce carbon monoxide poisoning, or a fire.
Tents and sleeping bags from Argos aren't exactly going to be of the highest quality; they're designed for a festival not Britain in January.
It's all desperately sad, and indicative of a couple who cannot put their baby's needs first.