Fly/sawfly??

Fly/sawfly??

Observation - Fly/sawfly?? - UK and Ireland. Description: Only one snatched picture of this curious insect, about 12mm long (excluding the curious 7 or 8 segmented antennae). Having trouble even getting this to the right order - any ideas?

Only one snatched picture of this curious insect, about 12mm long (excluding the curious 7 or 8 segmented antennae). Having trouble even getting this to the right order - any ideas?