Just a further update - I really came unstuck last night with my concept of a Rumsfeldesque lean army (4 heavy, 3 archers, 2 cav, 2 dogs, 1 general). I was sieging Capua when the Scipii came up with 2 full stacks. Nothing in MTW had prepared me for battling 40 units, mainly of heavy infantry, all deployed on the map at the same time. My elite force did brilliantly, driving most of them away but there were too many and my men died virtually to a man - including the high command general. First hard lesson - bring a full stack when invading Roman heartlands!

I still think the lean army can probably survive in the field against single stacks twice its size. But besieging increases the greater likelihood of being attacked simultaneously by two stacks (one reinforcing) and thus outnumbered 4:1 which is too much. This makes besieging rather risky unless you have driven away nearby field armies. Rather nice - it seems fairly realistic (Alesia, anyone?).

Other learning points - legionary cav are rather good. Elsewhere, I was forced to reluctantly charge one head first into a unit of auxilia spearmen in a city forum and did not lose a man! Although on reflection, maybe the learning point is that auxilia are rather bad...