Great idea to check where the builders bag came from (to link with a purchase//address).
I believe someone earlier suggested the idea of the body being in concrete. But according to the link below, lime and concrete actually delay the decomposing - not that the perpetrator might know that! So it is possible and would certainly be very difficult to break up and examine a body.
My thought was the builders bag was just a convenient large thing to wrap a body in to disguise it, in a location where other junk/builders rubble has been tipped (we don't know that - that is MOO, speculation based on the info that the area was known for fly tipping).
Assume it's ok to link this academic article -if not then please remove or let me know!
"Of the many factors that affect buried remains, one that is particularly pertinent to forensic studies is the inclusion of lime in a burial. It would seem that assailants believe that lime accelerates the decomposition of soft tissues in buried remains. However, observations by forensic investigators suggest that lime actually tends to slow the decomposition of buried remains"
The Effect of Cement and wrapping on the Decomposition rate of the Rabbit Carcasses