I need to know what capacitor a 70 watt SON lamp would use with a Philips BSN70L407-ITS 70W SON/MH/CDM Ballast & Philips ignitor model: SN57 HID Ignitor 50-70W.

Here the capacitor serves two purposes:
- Power factor correction, for that the optimum value is 12uF. But  PFC is not needed for lamp operation, unless you have large installation.
- Current path for the rather high frequency ignition pulse primary current (valid for semiparallel ignitors). But the capacitance is by far not critical then, anything above ~1uF is enough, but the wiring loop capacitor-ballast-ignitor-capacitor have to be kept as short as possible (so these three components as close together as possible, interlinked with short wires).
The superimposed ignitors do not need this functionality at all.

look at some of these photos of this street lantern running a 70 watt SON http://www.streetlightonline.co.uk/MI55_photographs.htm I can not make out the pic with the gear in it. What do you think?

Ihaven't seen the ignitorthere, it is a selfstarting lamp?

It appears to be.