The whole thing turned into a bit of a farce. I had one army made up of Hellenic/Celtic and naked spearmen and a reinforcing army of HA.

The spearmen attacked as a unified broad front and connected with all of the enemy units (including the FM's). The enemy infantry decided to bunch in the middle of my line allowing a double envelopment (whilst individual units of spearmen followed each FM around the field). The arrival of the HA led to the collapse of the infantry in the centre and then the FM died in melee one by one.

In some respects it was a bit disappointing - had the AI used some intelligence and used repeated charges with their FM from behind against my line (which they could have done prior to the HA arriving in great numbers) then the result would have been a lot more bloody (as it was a still took a fair few casualties - most FM died when they broke and then were hunted down).