Can a fellow PS-Fiver or generally knowledgeable person advise me on how to get the 'Stomp' move to work reliably ~ as I almost invariably get either the ‘Light Combo’, ‘Quick Punch’ or ‘Front Kick’ instead ~ with the ‘Front Kick’ being the most likely result.

Now obviously it is much more reliable to just go for the ‘Front Kick’ by way of a single tap and press of the L1 button, but just chain-sawing and front-kicking minoris Tyranids to get to the majoris types (whilst my grapple is charging or targetting) is nowhere near as effective as the ‘Stomp’ ~ which only results for me once or twice a mission currently

I can do each of the other moves when I go for them in particular'; just not the stomp one other than by chance it seems once or twice a mission. 

Sounds like you’re holding the attack button for too long and so doing the other heavy attack combos instead. It needs to be tap, tap, tap, hold. You could also be pressing too fast - you need delay to a degree,  in conjunction with the character animations. I suggest you practice in the sparring area. Also bonus tip - if you ever use the double stomp you need to press the attack button before the first stomp has finished (not clear in the description).

Thank you so much for your advice on needing to delay to a degree in each case with the 'tap, tap, tap and press' pacing ~ as I am now getting stomps down about 75 percent of the time currently in the sparring arena.  Your advice is very much appreciated, and I will certainly keep in mind the tip for the second stomp once I have mastered the first.