i'd say the filter was a must mate, i'd also go for a walk round with just the lamp and re familiarise our self with the land scape i found distances look a lot different at night, your best off walking and having a quick lamp sweep the feild turn it off then walking a bit more, try not to over lamp, obviously if you see something get settled down make a mentle note of where it was, put the lamp on last second take the shot, then lamp off move on when you've collected it, hope this is of some help to you, have fun and good luck. 
  
gary