A pars fracture of L5 occurs typically in about 5% of the population. See the section under isthmic spondylolisthesis under lumbar spine. Also typical is the ability to tolerate the back pain this fracture generates until one of two things happen. Either the disc at L5-S1 will disintegrate and the patient will develop significant lower back pain or the L5 vertebral body will collapse on the L5 nerve roots and leg pain will occur.
The injections you underwent were appropriate as some patients can improve from these. The MBBS was probably medial branch blocks, an injection used to see if you were a candidate for facet rhizotomies (see the section under facet injections). These are not typically very effective for this diagnosis.
Surgery is normally very successful for this disorder. See the section under TLIF to understand what surgeries can be used to alleviate the pain and stabilize this slip.