Thanks for the sprites GraPhiX - much appreciated.
There seems to be other parameters to play with (I haven't tested these - educated guess as to what they do from other work):
explosionType=99 (custom explosion)
explosionName=8x8spritesheet.dds (the explosion sprite sheet in same folder as projectile.txt)
explosionSize= (size of explosion decal above when rendered)
explosionLightFlag= 1 or 0 (give spotlight illumination along with explosion event
explosionSmokeName=8x8smokesheet.dds (the smoke sprite sheet that plays slightly after and behind main explosion, place in same folder as projectile.txt)
explosionSmokeSize = (size of smoke decal above when rendered)
explosionSparksCount=0 to 5 (debris style sparks alongside explosion)
explosionSparksSize = (size of sparks above)
PS - no one noticed what happened to the terrain about 25 seconds into my vid with one of the explosions
Cheers.