I very rarely post and sorry it's a long one, but please read!
The temperature in Bury st Edmunds on the 23rd and 24th September reached lows of 7 and 10 degrees, respectively. Had his body temperature acclimatised he would be hypothermic; but assuming his body temp had at least started to drop while outside, exacerbated by spending at least an hour stationary, sleeping, with alcohol induced dilation of blood vessels, he may become mildly hypothermic.
When he woke in that doorway, due to a text or needing the toilet, he would most likely be even more tired having possibly woken from a phase of deep sleep. He could also be more drunk, depending on how much alcohol he had consumed. He could have been confused and disoriented if mild hypothermia had set in.
Under these circumstances, I think it is possible that a scenario unfolded akin to this: He checks the time on his phone and places it on top of a bin while he prepares to urinate. After having looked around he worries he may be seen on CCTV here so he wanders to the loading bay in Short Brackland instead, forgetting his phone. This loading bay was not forensically tested. I would like to know if there was a waste bin that would have looked similar that night to the one on which I presume he left his phone. Could he have been confused enough to believe he had dropped his phone in a bin in the second loading bay, noticing it was no longer on top, but believing it to be the same bin as the one in the first loading bay? He could have reached in to get it, fallen in face first and died from positional asphyxia.