as almost all non traumatic musculoskeletal pain is a referred problem I cannot understand your confusion, it is quite clearly a referred pain phenomenon. Mobilise L5 thoroughly and observe the changes to this pain problem. It is by no means necessary in the diagnosis of a referred pain event to reproduce the pain with active and or passive movements of any relevant structure. It is a bonus if you can observe the connections prior to treatment. Simply apply mobilisation to L5 /s1 thoroughly, this may take four or five minutes , . Dural stretches of the sciatic trunk will be of value also.
The give away signs are the back pain that occurrs at the time or around the time of increased heel pain. It is more the case of attempting to prove that these common problems are not referred than to do the reverse.
Cheers
Geoff