How to fit things to a Bungaroosh wall
Accessing any substrate as jamietre suggests is probably the best bet if possible. Otherwise Tester101's epoxy will be best. The sort of shelf you want places fasteners in withdrawl, which is a difficult load to resist in marginal materials such as bungaroosh. If you attempt an epoxied anchor, you should attempt to undercut the hole with a small hard chisel to further help resist withdrawl forces. This way, you are not entirely dependent on the epoxy bond strength. Unfortunately, you still need to depend on the shear strength of bungaroosh anyway, which is marginal at best.

The deeper the hole, and the greater the undercut without damaging the surrounding wall material will yield the strongest anchorage.