APU start considerations didn't really apply in this situation as they were so low at the time and visual with the surrounding, so I would of (if I had thought of it) started the APU. The problem comes when you are at cruising altitude with a double engine failure, getting the APU up and running ASAP is advantageous as it will allow you to keep the ADIRU's (within 7 minutes of GEN failure) up and running to give you route/airport info, will also allow some restoration of hyd and electrical systems. The problem will come if for some reason the APU doesn't start and you end up wasting most of your precious BAT time because at stated above when the airspeed gets to around 140kts the RAT will potentially failue leaving you purely on BAT power, this could be quite akward if at this stage you are not visual with the runway/landing site and you lose ALL instrumentation whilst IMC. Having said all of that I think that I would always have one go at an APU start (as recommended by the QRH) as you would have to be having a really bad day (again!!) for all the other stuff to go wrong as well. It is just important to understand the consequences of failed APU start attempts.