Hi:
Indeed D2 is facing the wrong way in the original schematic as it will short the transformer primary. As I suggested before, if you simply remove D2, and replace R3 with 100 Ohms, you should be able to get the circuit working. There is no need for D2 if driving directly from the output of the 555 since it can both sink and source current.
Sebi's circuit should also work. In this case, D2 IS required because, as mentioned before, the collapsing field in the transformer primary will induce potentially damaging voltage spikes.
Jem